RFR: 8371018: Remove unused CollectedHeap::fill_with_object

[Albert Mingkun Yang]

Trivial removing dead code.

Test: GHA

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Commit messages:
 - trivial-remove

Changes: https://git.openjdk.org/jdk/pull/28082/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28082&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371018
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28082.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28082/head:pull/28082

PR: https://git.openjdk.org/jdk/pull/28082