btrfs: sink wait_for_unblock parameter to async commit

There's only one caller left btrfs_ioctl_start_sync that passes 0, so we
can remove the switch in btrfs_commit_transaction_async.

A cleanup 9babda9f ("btrfs: Remove async_transid from
btrfs_mksubvol/create_subvol/create_snapshot") removed calls that passed
1, so this is a followup.

As this removes last call of wait_current_trans_commit_start_and_unblock,
remove the function as well.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>