Remove .boxshadow LESS mixin

Not needed, box-shadow is supported without vendor prefixes in modern
browsers. Old browsers probably shouldn't have shadows enabled anyway
because it reduces performance on slower devices (especially scrolling).

http://nerds.airbnb.com/box-shadows-are-expensive-to-paint
http://daneden.me/2011/11/a-non-scientific-box-shadow-experiment/

Change-Id: I32801030958fcff9cd108e1ffa6f66f93671abe0
Merge "Remove .boxshadow LESS mixin"