Remove MySQL support as an EE feature

Having MySQL support as an Omnibus EE feature sends the message that we support it since it's potentially something a customer would pay for. However, in https://docs.gitlab.com/ce/install/requirements.html#database we clearly state that we do not recommend it due to lacking features.

Discussion thread: https://gitlab.slack.com/archives/C0NFPSFA8/p1499975726506304

Final decision was:

• Remove MySQL support as an EE feature
• Mark it as deprecated in the relevant section of the docs https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2593

marked this issue as related to gitlab-org/omnibus-gitlab#2593 (closed)

changed the description

mentioned in merge request !6792 (closed)

marked the checklist item Mark it as deprecated in the relevant section of the docs https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2593 as completed

marked the checklist item Remove MySQL support as an EE feature as completed

mentioned in merge request !7027 (merged)

closed via merge request !7027 (merged)

mentioned in commit 48233acb