RFR (M): 8235907: Fix tests referencing UseParallelOldGC

[Thomas Schatzl]

Hi all,

      can I have reviews for this small change that removes uses of the
flag UseParallelOldGC from testing? With jdk15 this flag has been
obsoleted, so all tests using it error out.

There are follow-up RFEs to remove and eventually clean out parallel gc
serial old code, but this at least makes the failures go away.

CR:
https://bugs.openjdk.java.net/browse/JDK-8235907
Webrev:
http://cr.openjdk.java.net/~tschatzl/8235907/webrev/
Testing:
hs-tier1-5 (still running, almost complete)

Thanks,
    Thomas