Discussion: Remove Parallel=Subdir feature

[llxia]

Parallel=Subdir feature was introduced before we had actual parallelization (i.e., Parallel=Dynamic or NodesByIterations). At that time, we ran tests in parallel according to subdir, which was used for external tests only. With the actual parallelization (i.e., Parallel=Dynamic), we do not use Parallel=Subdir feature anymore. The external tests are updated to use Parallel=Dynamic. As far as I know, this Parallel=Subdir feature is no longer used. I would like to propose removing this feature from our code base.

By removing Parallel=Subdir feature, it will make Parallel logic more simple and easy to add the enhancements (such as #4093 and #3986)

[smlambert]

I think this seems like a wise course of action. We no longer need this feature and removing it removes complexity and potential tech debt of supporting an unused feature.