RFR: 8326171: Serial: Remove VerifyGCLevel

[Albert Mingkun Yang]

Simple removing a diagnostic JVM flag.

-------------

Commit messages:
 - s1-verify-level

Changes: https://git.openjdk.org/jdk/pull/17914/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17914&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326171
  Stats: 9 lines in 2 files changed: 0 ins; 5 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17914.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17914/head:pull/17914

PR: https://git.openjdk.org/jdk/pull/17914