RFR: 8253647: Remove dead code in os::create_thread() on Linux/BSD

[Zhengyu Gu]

Please review this small patch to remove dead code that is left behind by JDK-8078513.

Test:
- [x] tier1 on Linux 86_64

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Commit messages:
 - 8253647: Remove dead code in os::create_thread() on Linux/BSD

Changes: https://git.openjdk.java.net/jdk/pull/361/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=361&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253647
  Stats: 14 lines in 2 files changed: 0 ins; 14 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/361.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/361/head:pull/361

PR: https://git.openjdk.java.net/jdk/pull/361