btrfs: remove unused btrfs_fs_info::total_pinned

This got added 14 years ago in 324ae4df ("Btrfs: Add block group
pinned accounting back") but it was not ever used. Subsequently its
usage got gradually removed in 8790d502 ("Btrfs: Add support for
mirroring across drives") and 11833d66 ("Btrfs: improve async block
group caching"). Let's remove it for good!
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -823,8 +823,6 @@ struct btrfs_fs_info {
 	struct kobject *space_info_kobj;
 	struct kobject *qgroups_kobj;
 
-	u64 total_pinned;
-
 	/* used to keep from writing metadata until there is a nice batch */
 	struct percpu_counter dirty_metadata_bytes;
 	struct percpu_counter delalloc_bytes;