RFR: 8301446: Remove unused includes of gc/shared/genOopClosures

[Albert Mingkun Yang]

Simple removing unused includes.

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Commit messages:
 - cleanup-genoopclosure-header

Changes: https://git.openjdk.org/jdk/pull/12307/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12307&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301446
  Stats: 5 lines in 5 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12307.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12307/head:pull/12307

PR: https://git.openjdk.org/jdk/pull/12307