Remove MySQL references from development docs

What does this MR do?

I noticed the doc/development/testing_guide/best_practices.md still referenced the [run mysql] tags, etc. They no longer work, so I removed them, then realised I had better clean up the rest of doc/development !

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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