RFR: 8377141: G1: Remove unused local declaration in G1BarrierSetC2

[Albert Mingkun Yang]

Trivial removing dead code.

Test: tier1

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Commit messages:
 - g1-trivial-remove-local-var

Changes: https://git.openjdk.org/jdk/pull/29561/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29561&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377141
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29561.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29561/head:pull/29561

PR: https://git.openjdk.org/jdk/pull/29561