Removed the Mergeable check

Now that we are syncing all PRs from Piper with Copybara, there isn't a convenient workflow for adding per-language labels to the GitHub PR.  (Also even if we required this, Copybara sometimes closes the PR and makes a separate commit, instead of merging the PR).

Long-term we want the release script to bucket by language by automatically detecting which files were modified.  In the near-term, the onduty will need to do this manually at release time.

PiperOrigin-RevId: 485730509

.github/mergeable.yml

@@ -1,18 +0,0 @@
-mergeable:
-  pull_requests:
-    label:
-      and:
-        - must_exclude:
-            regex: '^disposition/DO NOT MERGE'
-            message: 'Pull request marked not mergeable'
-        - or:
-          - and:
-            - must_include:
-                regex: 'release notes: yes'
-                message: 'Include release notes: yes'
-            - must_include:
-                regex: '^(autotools|bazel|c#|c\+\+|cleanup|cmake|conformance tests|integration|go|java|javascript|objective-c|php|protoc|python|ruby|kotlin)'
-                message: 'at least a language label (e.g., c++, java, python). Or apply one of the following labels: autotools, bazel, cmake, cleanup, conformance tests, integration, protoc.'
-          - must_include:
-              regex: 'release notes: no'
-              message: 'Include release notes: no'