Hide pipeline retry button for Merge Train/Pipelines

Summary

We got feedback that pipelines for merge trains cannot be retried. This is because once the merge request has been dropped from the merge train, it has to create a new pipeline always.

In order to solve this issue quickly, we will hide the retry button.

Future: We should disable pipeline retry button in this case as UX improvement. We should add a tooltip notifying the user with the following text: TBD

/cc @ogolowinski @rverissimo