CoreTestRunner: Remove support for skipping execution of tests

Following on from previous changes in this bug which remove the
need for Runner implementations to explicitly support a
'skip test execution' mode this removes explicit support for it
from the TestNgRunner and related classes.

Bug: 30801147
Test: run cts modules CtsLibcoreTestCases CtsLibcoreOjTestCases
Change-Id: I16538c39714331358d1f57a8eb036440f7c9e089
5 files changed