git-gui: Correct resizing of remote branch delete dialog

git-gui: Correct resizing of remote branch delete dialog

The status field of the remote branch delete dialog was marked to
expand, which meant that if the user grew the window vertically
most of the new vertical height was given to the status field and
not to the branch list.  Since the status field is just a single
line of text there is no reason for it to gain additional height,
instead we should make sure all additional height goes to the
branch list.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>