gitlab: Drop backport-for-8.8 MR template

Its usefulness has long passed.

.gitlab/merge_request_templates/backport-for-8.8.md

-Thank you for help in maintaining GHC's stable branch!
-
-Please take a few moments to verify the following:
-
- * [ ] the issue that this backport fixes is milestoned for the target release.
-
-If you have any questions don't hesitate to open your merge request and inquire
-in a comment.
-
-/label ~backport
-/milestone %8.8.1