[GFS2] Remove unused variable

The go_drop_th function is never called or referenced.
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/incore.h

@@ -131,7 +131,6 @@ struct gfs2_bufdata {
 struct gfs2_glock_operations {
 	void (*go_xmote_th) (struct gfs2_glock *gl);
 	void (*go_xmote_bh) (struct gfs2_glock *gl);
-	void (*go_drop_th) (struct gfs2_glock *gl);
 	void (*go_inval) (struct gfs2_glock *gl, int flags);
 	int (*go_demote_ok) (struct gfs2_glock *gl);
 	int (*go_lock) (struct gfs2_holder *gh);