RFR: 8286628: Remove unused BufferNode::Allocator::flush_free_list

[Albert Mingkun Yang]

Simple change of removing an unused API.

Test: hotspot_gc

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Commit messages:
 - remove

Changes: https://git.openjdk.java.net/jdk/pull/8676/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8676&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286628
  Stats: 5 lines in 2 files changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8676.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8676/head:pull/8676

PR: https://git.openjdk.java.net/jdk/pull/8676