gfs2: Remove freeze_go_demote_ok

Before commit b77b4a48 ("gfs2: Rework freeze / thaw logic"), the
freeze glock was kept around in the glock cache in shared mode without
being actively held while a filesystem is in thawed state.  In that
state, memory pressure could have eventually evicted the freeze glock,
and the freeze_go_demote_ok callback was needed to prevent that from
happening.

With the freeze / thaw rework, the freeze glock is now always actively
held in shared mode while a filesystem is thawed, and the
freeze_go_demote_ok hack is no longer needed.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>