RFR: 8269433: Remove effectively unused ReferenceProcessor::_enqueuing_is_done

[Albert Mingkun Yang]

Simple change of removing effectively dead code.

-------------

Commit messages:
 - set-equeue

Changes: https://git.openjdk.java.net/jdk/pull/4605/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4605&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269433
  Stats: 12 lines in 3 files changed: 0 ins; 11 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4605.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4605/head:pull/4605

PR: https://git.openjdk.java.net/jdk/pull/4605