Eliminate duplicate 'gutter' class

This was previously needed when we applied 'editor-colors' to the
line numbers to give them an opaque background in hopes of supporting
sub pixel AA, but that dream is dead for now anyway. This makes the
gutter harder to style, so I'm nixing it.

src/gutter-component.coffee

@@ -19,7 +19,7 @@ GutterComponent = React.createClass
     {scrollHeight, scrollViewHeight, onMouseDown} = @props
 
     div className: 'gutter', onClick: @onClick, onMouseDown: onMouseDown,
-      div className: 'gutter line-numbers', ref: 'lineNumbers', style:
+      div className: 'line-numbers', ref: 'lineNumbers', style:
         height: Math.max(scrollHeight, scrollViewHeight)
         WebkitTransform: @getTransform()