RFR: 8284572: Remove unneeded null check in ReferenceProcessor::discover_reference

[Albert Mingkun Yang]

Simple change of removing an always-false if-check.

Test: tier1-6

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Commit messages:
 - ref-discovery-null

Changes: https://git.openjdk.java.net/jdk/pull/8157/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8157&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284572
  Stats: 9 lines in 1 file changed: 2 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8157.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8157/head:pull/8157

PR: https://git.openjdk.java.net/jdk/pull/8157