gfs2: simplify finish_xmote

As a follow-up to commit a431d492 ("gfs2: Fix request cancelation
bug"), it turns out that any call to finish_xmote() is always followed
by a call to run_queue(), either

 * directly when glock_work_func() calls finish_xmote() before calling
   run_queue(), or

 * indirectly when do_xmote() calls finish_xmote() before calling
   gfs2_glock_queue_work(), which queues a call to glock_work_func() in
   work queue context,

so remove the code in finish_xmote() that duplicates the functionality
of run_queue().

In addition, the code this commit removes is missing a check for the
GLF_DEMOTE flag which indicates that no further promotes should be
performed, so if that code didn't get removed, that check would have to
be added.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>