RFR: 8293210: G1: Remove redundant check in G1FreeHumongousRegionClosure

[Albert Mingkun Yang]

Simple change of removing unnecessary code.

Test: hotspot_gc

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Commit messages:
 - g1-entail-2

Changes: https://git.openjdk.org/jdk/pull/10119/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10119&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293210
  Stats: 5 lines in 2 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10119.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10119/head:pull/10119

PR: https://git.openjdk.org/jdk/pull/10119