[jdk11u-dev] RFR: 8261652: Remove some dead comments from os_bsd_x86

[Vladimir Kempik]

Please review this backport of 8261652.
Didn't apply clean due to different location of comment.
This patch removes some dead comment about older macos and removes double inclusion of pthread.h
Comment location is slightly different from jdk17, but since it is a comment I would call this effectively clean backport.
This is a soft prerequest for macarm.

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Commit messages:
 - Backport 28163a9e9840249e04384ce4e795418bd3715481

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/11/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=11&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261652
  Stats: 11 lines in 1 file changed: 0 ins; 11 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/11.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/11/head:pull/11

PR: https://git.openjdk.java.net/jdk11u-dev/pull/11