Remove the `OpenProjectService` integration

About

This issue came out of this discussion #329410 (comment 563428794).

The OpenProjectService integration is incomplete #15847 (comment 388912720). As ~"group::ecosystem" are unlikely to complete it at present #15847 (comment 422065412) we should remove it.

Note, this service may get renamed to Integrations::OpenProject as part of #201855 (closed) simply to be able to delete the project_services directory.

Proposal

We should revert !28012 (merged), or if the MR is unrevertable, remove all of the relevant code.

We should also search through the codebase for OpenProject and open_project and clean up any references.

We should update the description of #15847 with this information and link to the original MR so in future, the work can be jump-started if a community contributor wishes to work on it.