SSP-1766 Key and constraint cleanup for external_student_test

Two sets of dbms-specific changesets for clearing and recreating
external_student_test keys/constraints. As noted in comments, this
change does try to take into account the fact that a different
approach had already been attempted in Linux/PG CI. But still likely
to have to deal with some manual cleanup in dev envs, at a minimum
with b/c of some md5 changes in the 'add discriminator column to
external_student_test' changeset.

Removes Java-coded Liquibase generators for deleting unnamed
keys/constraints. Ended up being able to do that from the XML.