Stop using mergify merge queue feature (Qiskit#9807)

Due to recent changes in the mergify configuration/service the merge
queue feature of mergify is no longer working as expected. It's not
clear if this just a temporary bug in mergify, a longer term change in
behavior (which requires all PR authors to be registered on mergify), or
some incompatibility between the github api and mergify. However,
because we're no longer able to rely on it we've moved to using github's
native merge queue feature [1][2] instead. Since this new merge queue
would conflict with mergify's this commit removes the merge queue
configuration from the mergify configuration. The configuration for
stable backporting is left in place because github doesn't natively
offer that feature and it appears to still work fine.

[1] https://github.blog/changelog/2023-02-08-pull-request-merge-queue-public-beta/
[2] https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue

.mergify.yml

@@ -1,19 +1,5 @@
-queue_rules:
-  - name: automerge
-    conditions:
-      - check-success=Qiskit.qiskit-terra
-
+---
 pull_request_rules:
-  - name: automatic merge on CI success and review
-    conditions:
-      - check-success=Qiskit.qiskit-terra
-      - "#approved-reviews-by>=1"
-      - label=automerge
-      - label!=on hold
-    actions:
-      queue:
-        name: automerge
-        method: squash
   - name: backport
     conditions:
       - label=stable backport potential