[DBAL-54] Cannot drop schema using IDENTITY strategy in PostgreSQL

DBAL-54 was fixed in commit 3d55dc8eec235c09d64b061456295a1951299496 by
changing SQL queries order. However it was later reintroduced.

This patch uses a different, more future-proof way, approach by using
DROP SEQUENCE ... CASCADE syntax, and thus does not depend on queries order.