Build/Test Tools: Remove the check for changes to version-controlled files in the Test Old Branch workflow.

This workflow does not perform a `git checkout` and is only used to dispatch workflows for old branches, so this check is not needed here.

Merges [51498] to the 5.8 branch.
Fixes #53799.

git-svn-id: https://develop.svn.wordpress.org/branches/5.8@51502 602fd350-edb4-49c9-b593-d223f7449a82

.github/workflows/test-old-branches.yml

@@ -67,6 +67,3 @@ jobs:
               workflow_id: '${{ matrix.workflow }}',
               ref: '${{ matrix.branch }}'
             });
-
-      - name: Ensure version-controlled files are not modified during the tests
-        run: git diff --exit-code