RFR: 8273186: Remove leftover comment about sparse remembered set in G1 HeapRegionRemSet

[Thomas Schatzl]

Hi all,

  please review this trivial one-line obsolete comment about sparse remembered sets which do not exist any more.

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - Remove obsolete

Changes: https://git.openjdk.java.net/jdk/pull/5319/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5319&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273186
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5319.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5319/head:pull/5319

PR: https://git.openjdk.java.net/jdk/pull/5319