Remove PrebuildStatusMaintainer (last non-local writer to messagebus besides ws-manager-bridge)

`PrebuildStatusMaintainer` is the last writer to non-local messagebus (besides ws-manager-bridge). In order to disable cross-cluster messages, we should disable it.

Note that this comes with a minor feature removal: We no longer push prebuild-statuus to GitHub commits.

Draft PR: https://github.com/gitpod-io/gitpod/pull/7284