Remove unused icons from sprite

Some icons in our sprite do not appear anywhere in the codebase.

By removing those unused icons we have a gain of 84KB (6KB gzipped) for the CSS.

### Removed icons

- arrow-large-down
- arrow-small-down
- close-x
- comment-count-bubble
- default-witness-avatar
- image-inverted
- image-white
- more-arrow
- tool


### Before

Metrics for common/app/assets/stylesheets/global.min.css
>> Total rules: 550
>> Total selectors: 863
>> Average selectors per rule: 1.6
>> File size: 407.39 Kb
>> GZip size: 46.70 Kb

Metrics for common/app/assets/stylesheets/head.min.css
>> Total rules: 380
>> Total selectors: 613
>> Average selectors per rule: 1.6
>> File size: 40.45 Kb
>> GZip size: 11.14 Kb

### After

Metrics for common/app/assets/stylesheets/global.min.css
>> Total rules: 530
>> Total selectors: 829
>> Average selectors per rule: 1.6
>> File size: 323.64 Kb
>> GZip size: 40.93 Kb

Metrics for common/app/assets/stylesheets/head.min.css
>> Total rules: 371
>> Total selectors: 604
>> Average selectors per rule: 1.6
>> File size: 39.80 Kb
>> GZip size: 11.00 Kb

![Fuck yeah](http://i.imgur.com/d7TBk.gif)