Fix HSQLDB dialect for non-existing constraints.

This commits relates to https://hibernate.atlassian.net/browse/HHH-7002.
It basically removes the explicit constraints dropping, and uses
cascading instead.

As HSQLDB requires to put CASCADE last, but fortunately accepts IF
EXISTS also before tablename, we just put the IF EXISTS *before* the
tablename, and CASCADE after it. And there you go.