btrfs: qgroup: remove obsolete fs_info members

The commit fcebe456 ("Btrfs: rework qgroup accounting") reworked
qgroups and added some new structures. Another rework of qgroup
mechanics e69bcee3

 ("btrfs: qgroup: Cleanup the old
ref_node-oriented mechanism.") stopped using them and left uncleaned.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>