JDK9 Support: Remove deleted GC flags

[ash211]

Compare these two links:

• "Options That Won’t Work " section of https://www.azul.com/jdk-9-pitfalls-for-the-unwary/
• args we set at https://github.com/palantir/sls-packaging/blob/2.4.0/gradle-sls-packaging/src/main/groovy/com/palantir/gradle/dist/service/tasks/LaunchConfigTask.groovy#L34

We might need to remove:

• PrintGCDateStamps
• UseGCLogFileRotation
• GCLogFileSize
• NumberOfGCLogFiles

Possibly there are others as well, looking at JEP 214 on that page.

cc @schlosna

[schlosna]

Yeah, I expect that we need to allow selection of Java 8 or 9, and derive appropriate default flags for each. We'll likely need to enable the module escape valves for a bit.