QmlJSDebugger: Removed the double-click feature from the color box

QmlJSDebugger: Removed the double-click feature from the color box

It was supposed to show a color dialog, but in practice it never worked
due to using show() rather than exec(). Removed it completely since it
was judged confusing and not very useful, since it is already possible
to copy the color by using the context menu.

Task-number: QTCREATORBUG-3878
Reviewed-by: Kai Koehne