Remove code replacing core.jar -> core-libart.jar.

This code seems obsolete. If it was still needed, it would
have to replace core.jar with core-oj.jar instead; the boot
class path starts with core-oj.jar, not core-libart.jar.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I5dd6097894cd72c27b87dd572e6e65203ad23ff4