Remove useless class

Hi there! The `.editor-colors` selector defines `color` and `background-color`, but those values have less precedence than those for `.editor.mini`. So using all three classes together makes `editor-colors` useless.

I noticed that the current code with `editor-colors` conflicts with [Zen](https://github.com/defunkt/zen), making the background color in Zen mode incorrect.

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A more elaborated refactor might be using `TextEditorView(mini: true)` subviews instead of `input` elements, in the same way that [SelectListView](https://github.com/atom/atom/blob/master/src/select-list-view.coffee#L37) does, and then change the way we read the settings to something like [this](https://github.com/atom/atom/blob/master/src/select-list-view.coffee#L160-L161). But, perhaps, all this would be just overkill.