ci/mergify: drop

We no longer use mergify, so drop the config file.

.github/mergify.yml

@@ -1,31 +0,0 @@
-# Essential reading on condition syntax:
-# https://docs.mergify.com/configuration/conditions/#testing-and-debugging-conditions
-
-# https://docs.mergify.com/merge-queue
-queue_rules:
-  - name: default
-    # Account used for pushing rebased updates. Default is {{author}},
-    # but that doesn't work when they haven't logged into Mergify Dashboard:
-    # https://github.com/Mergifyio/mergify/issues/5075
-    update_bot_account: nix-infra-bot
-    update_method: rebase
-    merge_method: fast-forward
-    merge_conditions:
-      - check-success = buildbot/nix-build
-    queue_conditions:
-      - or:
-          # Allow queuing PRs that have been approved
-          - "#approved-reviews-by >= 1"
-
-          # Allow queuing update PRs without approval
-          - and:
-              # We need a double-negative, because mergify only has "any" conditions on list types...
-              #
-              # So we check if "any file does not match the regex", using the `-` prefix operator.
-              # The regex matches anything except `flake.lock` and `generated`, using a negative lookahead `(?!)`
-              #
-              # After canceling out the double-negative, the condition below is true when
-              # _all_ files match either `^flake\.lock$` or `^generated/`
-              - "-files ~= ^(?!flake[.]lock$|generated/)"
-              # Also check the PR was opened by the github-actions bot
-              - author = github-actions