RFR: 8330670: Obsolete ScavengeBeforeFullGC

[Albert Mingkun Yang]

Obsolete a gc product flag.

Test: tier1-3

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Commit messages:
 - remove-jvm-flag

Changes: https://git.openjdk.org/jdk/pull/18861/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18861&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330670
  Stats: 25 lines in 7 files changed: 1 ins; 21 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18861.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18861/head:pull/18861

PR: https://git.openjdk.org/jdk/pull/18861