[GFS2] Remove drop of module ref where not needed

In an earlier patch "[GFS2] fix file_system_type leak on gfs2meta mount"
we removed the code to grab a ref to the module which was not needed
(since we know that the module cannot be unloaded at that time) so
this patch removes the code to drop that reference.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>