[8u-dev]: RFA (XS): JDK-8217520: Remove vm.opt.MaxGCPauseMillis == "null" from TestOldGenCollectionUsage.jav

[Hohensee, Paul]

Please approve this trivial backport of JDK-8217520 to 8u.

JBS: https://bugs.openjdk.java.net/browse/JDK-8217520
Webrev: http://cr.openjdk.java.net/~phh/8217520/webrev.00/
Review thread: https://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2019-January/024625.html

This is a one-line patch that applies cleanly.

Thanks,

Paul