Remove unnecessary dependencies in sub-modules

[PoeppingT]

In particular, we need to remove dependencies that may introduce perception of vulnerabilities. For example, we've received dependabot PRs like #192 which upgrades the declared postgresql dependency in the rds-bootstrap custom resource, despite the fact that that dependency is used nowhere in the resource.

Other information

[brtrvn]

That dependency is used if the database engine SaaS Boost is bootstraping during tenant onboarding is a PostgreSQL database. Same is true for the other database drivers. We determine the correct driver at runtime and load it with java.sql.DriverManager.