RFR: 8316420: Serial: Remove unused GenCollectedHeap::oop_iterate

[Albert Mingkun Yang]

Trivial removing dead code.

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

Commit messages:
 - s1-remove-oop-iter

Changes: https://git.openjdk.org/jdk/pull/15785/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15785&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316420
  Stats: 6 lines in 2 files changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15785.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15785/head:pull/15785

PR: https://git.openjdk.org/jdk/pull/15785