[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>

diff --cc fs/gfs2/glock.c
Simple merge

diff --cc fs/gfs2/incore.h
Simple merge