Clean up defining classes in ClassLinker.

Change ClassLinker::FindClassInBaseDexClassLoaderClassPath()
to separate class def lookup from class definition to avoid
deep call hierarchy and clean up an obsolete comment.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 148412019
Change-Id: If3d3900c06579a38065b24cff61f8c256209690d