Remove dropbutton css after it gets committed to CTools

Matches the CTools issue: #1557662: Add the dropbutton css from Views.

Quoting myself:

When the dropbuttons were styled for Views, the CSS was placed in Views and not CTools.
This greatly reduces their usefulness, since they are unstyled by default unless the Views admin CSS is specifically added. Since they are a new UI pattern, we definitely want them to look the same everywhere.

So let's move the Views CSS into CTools.