po: stop storing libvirt.pot in git

Storing the libvirt.pot file is a bad idea because it is an
automatically generated file. Most patches will invalidate the stored
libvirt.pot file by changing line numbers or introducing/removing files
with translatable content.

Anyone working with a libvirt GIT checkout who needs the libvirt.pot is
better served creating a fresh copy with "make libvirt.pot".

libvirt.pot is still included in the release dists, so those building
from tarballs see no change in behaviour.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>