Don't tell superview to redraw on mouse enter / exit

This is really slow, and it didn't even fix the clipping problem
that was originally seen in 9c44a70b1e69f2fa72f16fd3e9cc5dd794478d95.

A little concerned this is going to end up causing a regression, but
I suppose this just means twIRCk will need a bit more user testing
before the next update is submitted.