Remove the `OpenProjectService` integration
This issue came out of this discussion #329410 (comment 563428794).
Note, this service may get renamed to
Integrations::OpenProject as part of #201855 simply to be able to delete the
We should revert !28012 (merged), or if the MR is unrevertable, remove all of the relevant code.
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.