Remove "mysql.implementation" configuration

Summary:
Ref T11044. Fixes T10931. This option has essentially never been useful for anything, and we've picked the best implementation for a long time (MySQLi if available, MySQL if not).

I am not aware of any reason to ever set this manually. If someone comes up with some bizarre but legitimate use case that I haven't thought of, we can modularize it.

Test Plan: Browsed around. Grepped for mysql.implementation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10931, T11044

Differential Revision: https://secure.phabricator.com/D16909