RFR: 8264513: Cleanup CardTableBarrierSetC2::post_barrier

[Aleksey Shipilev]

There are few stale comments after CMS removal. We can also clean up some coding, while we are at it.

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Commit messages:
 - 8264513: Cleanup CardTableBarrierSetC2::post_barrier

Changes: https://git.openjdk.java.net/jdk/pull/3285/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3285&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264513
  Stats: 20 lines in 1 file changed: 5 ins; 5 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3285.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3285/head:pull/3285

PR: https://git.openjdk.java.net/jdk/pull/3285