RFR: 8265414: Variable assigned but not used in G1FreeHumongousRegionClosure

[Albert Mingkun Yang]

Trivial change of removing an unused field.

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Commit messages:
 - proxy-set

Changes: https://git.openjdk.java.net/jdk/pull/3565/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3565&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265414
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3565.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3565/head:pull/3565

PR: https://git.openjdk.java.net/jdk/pull/3565