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>

drivers/md/dm-clone-metadata.c

@@ -276,12 +276,6 @@ static inline int superblock_read_lock(struct dm_clone_metadata *cmd,
 	return dm_bm_read_lock(cmd->bm, SUPERBLOCK_LOCATION, &sb_validator, sblock);
 }
 
-static inline int superblock_write_lock(struct dm_clone_metadata *cmd,
-					struct dm_block **sblock)
-{
-	return dm_bm_write_lock(cmd->bm, SUPERBLOCK_LOCATION, &sb_validator, sblock);
-}
-
 static inline int superblock_write_lock_zero(struct dm_clone_metadata *cmd,
 					     struct dm_block **sblock)
 {