[GFS2] go_drop_bh is never used, so remove it

The ->go_drop_bh function is never used, so this removes it and the single
caller,

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

fs/gfs2/glock.c

@@ -953,9 +953,6 @@ static void drop_bh(struct gfs2_glock *gl, unsigned int ret)
 		spin_unlock(&gl->gl_spin);
 	}
 
-	if (glops->go_drop_bh)
-		glops->go_drop_bh(gl);
-
 	spin_lock(&gl->gl_spin);
 	gl->gl_req_gh = NULL;
 	gl->gl_req_bh = NULL;