btrfs: rip out btrfs_space_info::total_bytes_pinned

We used this in may_commit_transaction() in order to determine if we
needed to commit the transaction.  However we no longer have that logic
and thus have no use of this counter anymore, so delete it.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>