ci: remove mergify.yml as it's no longer used

.github/mergify.yml

@@ -1,37 +0,0 @@
-pull_request_rules:
-  - name: Merge lockfile updates
-    conditions:
-      - "title=Update lockfile.json"
-    actions:
-      review:
-        type: APPROVE
-        message: Automatically approving lockfile updates
-      merge:
-        method: merge
-
-  - name: Prepare for merge
-    conditions:
-      - and:
-        - "-draft"
-        - "#approved-reviews-by=1"
-        - "#review-requested=0"
-    actions:
-      comment:
-        message: |
-          This PR is ready to be merged, and will be in 1 day if nothing happens before.
-          If you want other people to review your PR, request their reviews.
-          If you don't want this PR to be merged now, mark it as a Draft.
-
-  - name: Merge on approval
-    conditions:
-      - and:
-        - or:
-          - "#approved-reviews-by>=2"
-          - and:
-            - "#approved-reviews-by=1"
-            - "updated-at>=1 day ago"
-        - "-draft"
-        - "#review-requested=0"
-    actions:
-      merge:
-        method: rebase