RFR (T) 8252652: Buggy looking null check in ServiceThread::oops_do()

[Coleen Phillimore]

Summary: Remove the null check.

Tested with jvmti tests and tier1 on all platforms.

open webrev at http://cr.openjdk.java.net/~coleenp/2020/8252652.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8252652

Thanks,
Coleen