Remove the @imports from bootstrap-slider.less

… because we don't want to edit anything bower_components, and gulp chokes on the missing dependencies.  If we're using bootstrap, bower, and less, then these are already available.

hth… let me know if I'm barking up the wrong tree!

tx for the project :)

Fixes:

```
[12:54:29] gulp-notify: [Error running Gulp] Error: 'variables.less' wasn't found in file /Users/snip/static/app/bower_components/seiyria-bootstrap-slider/less/bootstrap-slider.less line no. 8
```