RFR: 8280072: Remove unnecessary use of ParGCRareEvent_lock in Serial GC

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that removes an always-false clause in an assert? In more detail, that clause refers to `ParGCRareEvent_lock` that is never held in this code that is only called by serial gc.

Testing: gha, tier1-3

Thanks,
  Thomas

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Commit messages:
 - Initial version

Changes: https://git.openjdk.java.net/jdk/pull/7107/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7107&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280072
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7107.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7107/head:pull/7107

PR: https://git.openjdk.java.net/jdk/pull/7107