RFR: 8287150: Remove HeapRegion::block_start_const declaration without definition

[Thomas Schatzl]

Hi,

  please review this trivial change to remove an unused declaration without definition.

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - 8287150: Remove HeapRegion::block_start_const declaration without definition

Changes: https://git.openjdk.java.net/jdk/pull/8834/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8834&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287150
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8834.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8834/head:pull/8834

PR: https://git.openjdk.java.net/jdk/pull/8834