Remove skips for EnableCollection tests

These tests have been made less flaky.

Test: while ./art/tools/run-libjdwp-tests.sh --mode=host --test org.apache.harmony.jpda.tests.jdwp.MultiSession.EnableCollectionTest; do; done;
Test: while ./art/tools/run-libjdwp-tests.sh --mode=host --test org.apache.harmony.jpda.tests.jdwp.ObjectReference.EnableCollectionTest; do; done;
Test: while ./art/tools/run-libjdwp-tests.sh --mode=host --test org.apache.harmony.jpda.tests.jdwp.ObjectReference.IsCollectedTest; do; done;
Bug: 70958370
Bug: 69121056

Change-Id: Ic92d58d5c01559bf795327121a80952ff317cfae