RFR: 8289006: Cleanup from thread.hpp split

[Coleen Phillimore]

Remove extra include directives.  Tested with GHA.

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Commit messages:
 - 8289006: Cleanup from thread.hpp split

Changes: https://git.openjdk.org/jdk/pull/9252/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9252&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289006
  Stats: 7 lines in 2 files changed: 2 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9252.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9252/head:pull/9252

PR: https://git.openjdk.org/jdk/pull/9252