RFR: 8316973: GC: Pass external vm flags to TestDisableDefaultGC.java

[Leo Korinth]

There seems there is no need to strip external vm flags. The `@requires vm.gc=="null"` will handle the case when we iterate different GCs and we will then just skip the test.

Tested with:
`make run-test TEST=open/test/hotspot/jtreg/gc/arguments/TestDisableDefaultGC.java JTREG='RETAIN=all;VERBOSE=all;JAVA_OPTIONS=-XX:+UseG1GC'` -> total 0
`make run-test TEST=open/test/hotspot/jtreg/gc/arguments/TestDisableDefaultGC.java` -> success

started tier 1-5

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Commit messages:
 - 8316973: GC: Pass external vm flags to TestDisableDefaultGC.java

Changes: https://git.openjdk.org/jdk/pull/15931/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15931&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316973
  Stats: 8 lines in 1 file changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/15931.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15931/head:pull/15931

PR: https://git.openjdk.org/jdk/pull/15931