Btrfs: remove unused mutex from struct 'btrfs_fs_info'

The code using 'ordered_extent_flush_mutex' mutex has removed by below
commit.
 - 8d875f95da
   btrfs: disable strict file flushes for renames and truncates
But the mutex still lives in struct 'btrfs_fs_info'.

So, this patch removes the mutex from struct 'btrfs_fs_info' and its
initialization code.

Signed-off-by: Byongho Lee <bhlee.kernel@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ctree.h

@@ -1518,12 +1518,6 @@ struct btrfs_fs_info {
 	 */
 	struct mutex ordered_operations_mutex;
 
-	/*
-	 * Same as ordered_operations_mutex except this is for ordered extents
-	 * and not the operations.
-	 */
-	struct mutex ordered_extent_flush_mutex;
-
 	struct rw_semaphore commit_root_sem;
 
 	struct rw_semaphore cleanup_work_sem;