Remove mergeable action to allow copybara test reuse.

Our repo settings have already been updated to only allow merges from the BTR team and protobuf managers.  Except for some very specific circumstances, PRs should *never* be merged and should instead be imported via Copybara.

PiperOrigin-RevId: 506371362

.github/mergeable.yml

@@ -1,15 +0,0 @@
-mergeable:
-  pull_requests:
-    label:
-      and:
-        - must_exclude:
-            regex: '^disposition/DO NOT MERGE'
-            message: 'Pull request marked not mergeable'
-        - must_include:
-            regex: 'mergeable:force-allow'
-            message: 'Pull requests should not be merged directly and should instead
-              be handled by Copybara.
-
-              To enable Github merges, add the `mergeable:force-allow` label and get a second 
-              approval. This should almost never be used except for releases or as a break glass 
-              measure after discussing with the team.'