RFR: 8184134: HeapRegion::LogOfHRGrainWords is unused

[Albert Mingkun Yang]

Trivial change of removing unused code.

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Commit messages:
 - word

Changes: https://git.openjdk.java.net/jdk/pull/3671/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3671&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8184134
  Stats: 8 lines in 2 files changed: 0 ins; 6 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3671.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3671/head:pull/3671

PR: https://git.openjdk.java.net/jdk/pull/3671