btrfs: cleanup the useless DEFINE_WAIT in cleanup_transaction

When it was introduced in commit f094ac32 ("Btrfs: fix NULL pointer
after aborting a transaction"), it was not used.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>