Shenandoah: Avoid num of dead callback from weak processor in Shenandoah root verifier

This undesirable callback from root verifier breaks runtime/stringtable/StringTableCleaningTest.java test.

TEST_VM_OPTS="-XX:+UseShenandoahGC -XX:+UnlockDiagnosticVMOptions -XX:-ShenandoahVerify" make CONF=linux-x86_64-server-release run-test TEST=runtime/stringtable/StringTableCleaningTest.java