Simplified MySQL version checking.

Django used to check the version of MySQL before handling the first
request, which required:
- opening a connection
- closing it, to avoid holding it idle until the first request.

This code isn't necessary any longer since Django dropped support for
some versions of MySQL, and other database backends don't implement a
similar dance. For consistency and maintenability, remove it.

Reverts 4423757c.

Closes #18135.