RFR: 8304411: Remove unused CardTable::clear

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - remove-ct-clear

Changes: https://git.openjdk.org/jdk/pull/13075/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13075&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304411
  Stats: 8 lines in 2 files changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13075.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13075/head:pull/13075

PR: https://git.openjdk.org/jdk/pull/13075