RFR: 8375006: [Linux] Remove obsolete O_CLOEXEC check in os::open

[Jonas Norlinder]

Linux got support for O_CLOEXEC in 2.6.23. This version was released in October 9, 2007. OpenJDK does not support versions that old anymore so this logic can be removed.

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Commit messages:
 - Spacing
 - Remove obsolete checks

Changes: https://git.openjdk.org/jdk/pull/29168/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29168&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375006
  Stats: 39 lines in 1 file changed: 0 ins; 38 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29168.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29168/head:pull/29168

PR: https://git.openjdk.org/jdk/pull/29168