Stop advertising MySQL as an EE feature

Most of our customers are coming from CE, that means most of our customers are so happy with self-hosting a Postgres-backed GitLab that they want to pay us for more great features (If MySQL is a main decision driver here, then my assumption is wrong).

When they review the list of features that EE gets them and then they see MySQL support here https://about.gitlab.com/products/ and the perception is that it will be a better experience. So then they want to "Upgrade" to MySQL when they switch to EE.

Only after they switch will they discover that there are sometimes bugs that only creep up on MySQL and now and then a feature doesn't work quite as intended.

My suggestion is that while we can keep supporting it we should not list it as a "Product Feature" so that only the teams who truly know that they want to run MySQL and have great reasons for it, can find it in the docs and ask us about the level of support we provide for it.

/cc @lbot