RFR: 8277534: Remove unused ReferenceProcessor::has_discovered_references

[Albert Mingkun Yang]

Trivial change of removing dead code.

Test: build

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

Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/6496/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6496&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277534
  Stats: 12 lines in 2 files changed: 0 ins; 12 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6496.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6496/head:pull/6496

PR: https://git.openjdk.java.net/jdk/pull/6496