To propagate the deletion, you could do:
CREATE TRIGGER delete_unused_t2 AFTER DELETE on t1
FOR EACH ROW
BEGIN
IF old.t2_ID NOT IN (SELECT t2_ID FROM t1)
THEN BEGIN
DELETE FROM t2
WHERE t2.t2_ID = old.t2_ID;
END;
END IF;
END
If you want to do it periodically rather than in a trigger, you could do:
DELETE FROM t2
LEFT JOIN t1 USING (t2_ID)
WHERE t1_ID IS NULL