SSP-1766 Fix external_student_test PK cleanup

In environments that may have already run a previous PK recreate
changeset, the new changeset that tried to recreate it would error
out because there was nothing to drop it first.

The new PK recreate changeset would also error out even if you
hadn't previously run the "old" recreate changeset b/c the
cleanup changeset for *that* didn't target the PK correctly.