chore(Gruntfile): remove unused tasks

### Remove tasks
* remove concat tasks
* remove cssmin tasks

We need to remove the unused tasks since it's blocking the default grunt to run smoothly.

Here are the log when run the default grunt task:

```
grunt
Running "jshint:ci" (jshint) task
>> Report "results/jshint-result.xml" created.

Running "jshint:all" (jshint) task
>> 2 files lint free.

Running "clean:server" (clean) task

Running "concurrent:test" (concurrent) task
    
    Running "copy:styles" (copy) task
    
    
    Done, without errors.
    
    Elapsed time
    copy:styles  14ms
    Total        14ms
        
    Running "coffee:dist" (coffee) task
    
    Running "coffee:test" (coffee) task
    
    Done, without errors.
    
    Elapsed time
    coffee:test  10ms
    Total        10ms
        
    Running "compass:dist" (compass) task
    
    Running "compass:server" (compass) task
    
    Done, without errors.
    
    Elapsed time
    compass:dist    2s
    compass:server  595ms
    Total           3s
    
Running "autoprefixer:dist" (autoprefixer) task

Running "connect:test" (connect) task
Started connect web server on localhost:5050.

Running "karma:unit" (karma) task
INFO [karma]: Karma v0.10.2 server started at http://localhost:5000/
INFO [launcher]: Starting browser PhantomJS
INFO [PhantomJS 1.9.2 (Linux)]: Connected on socket 1dxyymy1DWAF5ld_IyKs
PhantomJS 1.9.2 (Linux): Executed 17 of 17 SUCCESS (0.23 secs / 0.042 secs)

Running "karma:e2e" (karma) task
WARN [karma]: Port 5000 in use
INFO [karma]: Karma v0.10.2 server started at http://localhost:5001/
INFO [launcher]: Starting browser PhantomJS
INFO [PhantomJS 1.9.2 (Linux)]: Connected on socket ahl86IPHDEyetAIFIygW
PhantomJS 1.9.2 (Linux): Executed 1 of 1 SUCCESS (0.309 secs / 0.055 secs)

Running "clean:dist" (clean) task
Cleaning dist/images...OK
Cleaning dist/views...OK

Running "useminPrepare:html" (useminPrepare) task
Going through  to update the config
Looking for build script HTML comment blocks

Configuration is now:

  cssmin:
  {}

  concat:
  {}

  uglify:
  { dist: { files: { '<%= yeoman.dist %>/scripts/scripts.js': [ 'dist/scripts/scripts.js' ] } } }

  requirejs:
  {}

Running "concurrent:dist" (concurrent) task
    
    Running "coffee:dist" (coffee) task
    
    Running "coffee:test" (coffee) task
    
    Done, without errors.
    
    Elapsed time
    coffee:test  18ms
    Total        18ms
        
    Running "svgmin:dist" (svgmin) task
    
    Done, without errors.
    
    Elapsed time
    svgmin:dist  262ms
    Total        262ms
        
    Running "copy:styles" (copy) task
    
    
    Done, without errors.
    
    Elapsed time
    copy:styles  15ms
    Total        15ms
        
    Running "htmlmin:dist" (htmlmin) task
    File dist/views/main.html created.
    
    Done, without errors.
    
    Elapsed time
    htmlmin:dist  17ms
    Total         18ms
        
    Running "imagemin:dist" (imagemin) task
    ✔ app/images/glyphicons-halflings-white.png (saved -4.89 kb)
    ✔ app/images/glyphicons-halflings.png (saved 2.17 kB)
    Minified 2 images (saved 1.56 kB)
    
    Done, without errors.
    
    Elapsed time
    imagemin:dist  26ms
    Total          26ms
        
    Running "compass:dist" (compass) task
    
    Done, without errors.
    
    Elapsed time
    compass:dist  596ms
    Total         597ms
    
Running "autoprefixer:dist" (autoprefixer) task

Running "copy:dist" (copy) task


Running "cdnify:dist" (cdnify) task
Going through  to update script refs

Running "ngmin:dist" (ngmin) task
ngminifying 

Running "uglify:dist" (uglify) task
>> Destination (dist/scripts/scripts.js) not written because src files were empty.

Running "rev:dist" (rev) task
dist/images/glyphicons-halflings-white.png >> fc83544a.glyphicons-halflings-white.png
dist/images/glyphicons-halflings.png >> 48f32d4c.glyphicons-halflings.png

Running "usemin:html" (usemin) task

Processing as HTML - dist/views/main.html
Update the HTML to reference our concat/min/revved script files
Update the HTML with the new css filenames
Update the HTML with the new img filenames
Update the HTML with data-main tags
Update the HTML with the data tags
Update the HTML with background imgs, case there is some inline style
Update the HTML with anchors images
Update the HTML with reference in input

Running "usemin:css" (usemin) task

Done, without errors.

Elapsed time
jshint:all          167ms
concurrent:test     5s
connect:test        58ms
karma:unit          1s
karma:e2e           673ms
useminPrepare:html  21ms
concurrent:dist     4s
uglify:dist         22ms
rev:dist            29ms
usemin:css          8ms
Total               11s

```