RFR: 8321814: G1: Remove unused G1RemSetScanState::_collection_set_iter_state

[Albert Mingkun Yang]

Trivial removing dead code.

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

Commit messages:
 - g1-trivial

Changes: https://git.openjdk.org/jdk/pull/17068/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17068&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321814
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17068.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17068/head:pull/17068

PR: https://git.openjdk.org/jdk/pull/17068