Remove stale mergify configuration

.mergify.yml

@@ -1,23 +0,0 @@
-pull_request_rules:
-  - name: instruct bors to merge PRs with passing tests and 2 approvals
-    conditions:
-      - "#approved-reviews-by>=2"
-      - "#changes-requested-reviews-by=0"
-      - "status-success=static_analysis"
-      - "-status-failure~=^build"
-      - -conflict
-      - label!=work-in-progress
-      - label!=blocked
-      - label!=no-mergify
-      - head~=^(?!release.*).*$
-      - base=dev
-    actions:
-      comment:
-        message: "bors r+"
-  - name: Delete branch if the pull request is merged
-    conditions:
-      - merged
-      - head~=^(?!release.*).*$
-    actions:
-      delete_head_branch:
-        force: false