RFR: JDK-8273877: os::unsetenv unused

[Matthias Baesken]

Please review this small change.
It seems the HS function os::unsetenv is not used any more for some time and could be removed.

Thanks, Matthias

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Commit messages:
 - JDK-8273877

Changes: https://git.openjdk.java.net/jdk/pull/5561/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5561&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273877
  Stats: 13 lines in 3 files changed: 0 ins; 13 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5561.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5561/head:pull/5561

PR: https://git.openjdk.java.net/jdk/pull/5561