RFR: 8272461: G1: remove empty declaration of cleanup_after_scan_heap_roots

[Albert Mingkun Yang]

Trivial change of removing an empty declaration.

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/5114/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5114&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272461
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5114.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5114/head:pull/5114

PR: https://git.openjdk.java.net/jdk/pull/5114