RFR: 8273033: SerialGC: remove obsolete comments

[Albert Mingkun Yang]

Trivial change of removing obsolete comments.

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/5267/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5267&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273033
  Stats: 7 lines in 1 file changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5267.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5267/head:pull/5267

PR: https://git.openjdk.java.net/jdk/pull/5267