RFR: 8378948: Remove unused local variable in RunnerGSInserterThread

[Albert Mingkun Yang]

Trivial removing dead code.

Test: GHA

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Commit messages:
 - gtest-trivial-unused-local-var

Changes: https://git.openjdk.org/jdk/pull/29992/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29992&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378948
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29992.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29992/head:pull/29992

PR: https://git.openjdk.org/jdk/pull/29992