RFR: 8298102: Remove DirtyCardToOopClosure::_last_explicit_min_done

[Albert Mingkun Yang]

Trivial change of removing effectively dead code.

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Commit messages:
 - s1-unused

Changes: https://git.openjdk.org/jdk/pull/11512/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11512&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298102
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11512.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11512/head:pull/11512

PR: https://git.openjdk.org/jdk/pull/11512