RFR: 8301744: Remove unused includes of genOopClosures.hpp

[Albert Mingkun Yang]

Trivial removing dead includes.

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Commit messages:
 - s1-remove-gen-oop-include

Changes: https://git.openjdk.org/jdk/pull/12404/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12404&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301744
  Stats: 2 lines in 2 files changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12404.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12404/head:pull/12404

PR: https://git.openjdk.org/jdk/pull/12404