Remove unused VARIABLE named EXTERNAL_REPO_BRANCH

[sophiaxu0424]

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.

[smlambert]

Thanks for raising this issue @sophiaxu0424 - during our Tuesday meeting we at first thought this is not used, but after looking, we see it was introduced as part of the work of an Outreachy project called Project Tether, to enable an external_custom target.

I believe we had a special Grinder job called Grinder_Tether that had the EXTERNAL_REPO_BRANCH parameter. I will recreate it, if it does not exist, to see where this feature ended up, before we decide if we can remove this variable.

[sophiaxu0424]

Grinder_Tether

Sure Shelley, thanks for your information for this. Will wait for your confirmation on it.

[sophia-guo]

See https://github.com/adoptium/aqa-tests/pull/2687