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>