RFR: 8296932: G1: Remove unused G1RootClosures::weak_oops

[Albert Mingkun Yang]

Simple change of removing dead code.

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

Commit messages:
 - g1-remove-root-weak-oop-closure

Changes: https://git.openjdk.org/jdk/pull/11134/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11134&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296932
  Stats: 5 lines in 3 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11134.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11134/head:pull/11134

PR: https://git.openjdk.org/jdk/pull/11134