Remove mysql specific code paths
We are going to drop support for MySQL with GitLab 12.1, see https://gitlab.com/gitlab-org/gitlab-ce/issues/52442#note_175326171.
We may want to consider actively cleaning up the code base and remove code paths that are specific to MySQL. The alternative is to clean up code as we touch it.
In any case opening this issue to gather opinions and see how we organize and communicate this.
cc @gl-database