Btrfs: remove no longer used member num_dirty_bgs from transaction

The member num_dirty_bgs of struct btrfs_transaction is not used anymore,
it is set and incremented but nothing reads its value anymore. Its last
read use was removed by commit 64403612 ("btrfs: rework
btrfs_check_space_for_delayed_refs"). So just remove that member.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>