gfs2: Remove LM_FLAG_PRIORITY flag

commit 0b93bac2 upstream.

The last user of this flag was removed in commit b77b4a48 ("gfs2:
Rework freeze / thaw logic").

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>