btrfs: un-deprecate ioctls START_SYNC and WAIT_SYNC

The two ioctls START_SYNC and WAIT_SYNC were mistakenly marked as
deprecated and scheduled for removal but we actualy do use them for
'btrfs subvolume delete -C/-c'. The deprecated thing in ebc87351
should have been just the async flag for subvolume creation.

The deprecation has been added in this development cycle, remove it
until it's time.

Fixes: ebc87351 ("btrfs: Deprecate BTRFS_SUBVOL_CREATE_ASYNC flag")
Signed-off-by: David Sterba <dsterba@suse.com>