btrfs: drop incompat bit for raid1c34 after last block group is gone

When there are no raid1c3 or raid1c4 block groups left after balance
(either convert or with other filters applied), remove the incompat bit.
This is already done for RAID56, do the same for RAID1C34.

Signed-off-by: David Sterba <dsterba@suse.com>