.mergify.yml: drop

The mergify configuration is outdated because job names have changed
significantly. Also the mergify experiment failed and we're on our way
towards GitHub merge queues, slowly but steadily.

Thus, we don't need that anymore.

.mergify.yml

@@ -1,21 +0,0 @@
-queue_rules:
-  # This rule is for https://docs.mergify.com/commands/queue/
-  # and can be triggered with: @mergifyio queue
-  - name: default
-    merge_conditions:
-      # all github action checks in this list are required to merge a pull request
-      - check-success=Attributes
-      - check-success=Check
-      - check-success=Outpaths (aarch64-darwin)
-      - check-success=Outpaths (aarch64-linux)
-      - check-success=Outpaths (x86_64-darwin)
-      - check-success=Outpaths (x86_64-linux)
-      - check-success=Process
-      - check-success=Request
-      - check-success=editorconfig-check
-      - check-success=label-pr
-      - check-success=nix-files-parseable-check
-      - check-success=nixfmt-check
-      - check-success=nixpkgs-vet
-    # queue up to 5 pull requests at a time
-    batch_size: 5