Remove unused VARIABLE named EXTERNAL_REPO_BRANCH

We have to remove this variable named `EXTERNAL_REPO_BRANCH` since it has been removed from the Jenkins pipeline configuration env settings, while it is still used in some test's tag_version wrongly, triggering passing error.

Search the test sources for `EXTERNAL_REPO_BRANCH` and its use in `tag_version`; confirm which references remain after its removal from the Jenkins pipeline environment. Remove the unused references and run the affected tests to verify they no longer trigger the reported error.

Written by the indexing model from the issue text.