btrfs: Remove BTRFS_SUBVOL_CREATE_ASYNC support

This functionality was deprecated in kernel 5.4. Since no one has
complained of the impending removal it's time we did so.

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