Remove GitLab < 12.0 deprecation site alert

Created by: courier-new

We will stop support GitLab versions < 12.0 in Sourcegraph 4.0. We have added a site alert for admins to warn them to upgrade if we detect they are using a code host on an unsupported version. Before we release 4.0, we should either remove the alert or promote it into a more aggressive error (e.g. "You're running an unsupported version of GitLab, plz upgrade now before something breaks").