RFR: 8279374: Remove unused JNIHandles::weak_oops_do

[Albert Mingkun Yang]

Trivial change of removing unused code.

Test: tier1

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

Commit messages:
 - jni-dead-code

Changes: https://git.openjdk.java.net/jdk/pull/6944/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6944&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279374
  Stats: 7 lines in 2 files changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6944.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6944/head:pull/6944

PR: https://git.openjdk.java.net/jdk/pull/6944