RFR: 8337721: G1: Remove unused G1CollectedHeap::young_collection_verify_type

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - g1-trivial

Changes: https://git.openjdk.org/jdk/pull/20438/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20438&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337721
  Stats: 11 lines in 2 files changed: 0 ins; 11 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20438.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20438/head:pull/20438

PR: https://git.openjdk.org/jdk/pull/20438