dm clone metadata: remove unused function

Fix the following clang warning:

drivers/md/dm-clone-metadata.c:279:19: warning: unused function
'superblock_write_lock' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>