Further remove code branches by database type (EE)

What does this MR do?

Another pass on EE similar to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31102

This change removes code where we branch out by type of database. In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29608, we already removed the if mysql?; ... else ... end parts. Here we take the other perspective and remove if postgresql?; ... else ... end branches.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/52442 but there's no other issue for it.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content