Don't set a background-pixel for our client frame windows

Previously we would get a short black flicker when a client closes. This
happened because first the client's window would get hidden and only a short
moment later would awesome react to this and close its own window. In the mean
time, the X server filled the frame window with its background-pixel which was
black.

Just removing the background-pixel means we get the default value which is None.
This means that the content will be left untouched and the client's window will
be visible for a moment longer.

Signed-off-by: Uli Schlachter <psychon@znc.in>