Remove non-mysql database support

[Matthew McPherrin]

Our build systems are overly complicated by our (not great) way of supporting multiple database engines.

I think we should remove postgres/h2, and check in the jooq generated Java.

That'll simplify building a lot. If there's somebody who wants to support another database later, then we could consider re-adding databases.

There will be additional burden on any developers making new migrations, because they'll have to check in the generated code, but we can provide a script to ensure that's done properly.

[Matthew McPherrin]

Postgres has been removed

[Mat Byczkowski]

H2 has been removed in #444.