RFR: 8298480: Remove unused KlassRemSet

[Albert Mingkun Yang]

Trivial removing of dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/11611/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11611&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298480
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11611.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11611/head:pull/11611

PR: https://git.openjdk.org/jdk/pull/11611