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.

By optimising the remaining icons (I re-exported some of them with inkscape) we have a total gain of **290 KB** (15 KB gzipped).

### 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: 534
> Total selectors: 851
> Average selectors per rule: 1.6
> File size: 115.03 Kb
> GZip size: 31.71 Kb

Metrics for common/app/assets/stylesheets/head.min.css
> Total rules: 371
> Total selectors: 597
> Average selectors per rule: 1.6
> File size: 39.67 Kb
> GZip size: 10.98 Kb

![Fuck yeah](http://i.imgur.com/d7TBk.gif)