Remove workaround for null klass during marking phase

It was expected that the issue occured due to missing fence during
object construction in certain cases. That issue is fixed and hence
removing the workaround.

Bug: 143109692
Bug: 173676071
Test: art/test/testrunner/testrunner.py
Change-Id: I058d1996dbfae46f28931f103c12f1404a758109
1 file changed