Remove support for MYSQL and MSSQL

[eighttrigrams]

Problem

We are confident to settle on PQSQL as our choice for a production DB.

Currently, we have tests disabled for MSSQL and MYSQL (deliberately, as #65 has shown that they would work in principle), which also means that at any given point in time we don't know whether they work. We also lost the ability to just merge in Gitea into Forkana due to numerous merge conflicts, so we can't rely on the fact that Gitea runs the test either.

Solution

Ditch support for MYSQL and MSSQL entirely.
Clean up not only testing, but all artifacts that can be found that pertain specifically to those to dbs.

[taoeffect]

I think we just need to disable those tests, we don't need to necessarily remove any code.

Does #73 do that? If so then I think it closes this issue.