Remove 'editor-colors' class from lines and gutter for mini editors

Having the editor-colors class on these elements was causing the theme
to be applied to lines in mini editors in the settings view, which
caused a black inset box to appear with dark syntax themes.

This was added to give the lines an opaque background which was supposed
to enable sub pixel anti-aliasing despite being on the GPU, but it
didn't seem to be working. Perhaps we can revisit this issue after the
Chrome 35 upgrade to see if sub pixel antialiasing works with opaque
backgrounds afterward.