remove the legacy system test runner

Removing the legacy system test runner in favor of the pytest runner will allow us to do a major cleanup in the way system tests are executed. Without having to support two different modes of operation, various issues are easier to resolve. It also enables the rewrite of the various perl scripts which the legacy runner uses into python in a way that is more in-line with the general pytest approach.

pytest runner has the capabilities to run both shell and python system tests and will continue to do so in the foreseeable future.

Related #3810