[keyserver] Get rid of all MySQL-specific query logic

Summary:
These conditionals were introduced during the MariaDB migration so the codebase could simultaneously support MySQL 5.7 and MariaDB 10.8.

Now that we've completed the migration, we're getting rid of all the old MySQL-specific query logic.

This diff closes ENG-1394.

Depends on D4834

Test Plan:

1. Flow
2. I ran keyserver locally and made sure things generally still worked

Reviewers: tomek, atul

Reviewed By: tomek

Subscribers: adrian, abosh

Differential Revision: https://phab.comm.dev/D4835