RFR: 8299845: Remove obsolete comments in DirtyCardToOopClosure::get_actual_top

[Albert Mingkun Yang]

Trivial removing stable comments.

-------------

Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/11918/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11918&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299845
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11918.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11918/head:pull/11918

PR: https://git.openjdk.org/jdk/pull/11918