RFR: 8365782: Remove unnecessary inclusion of <stdlib.h> in jfrOSInterface.cpp

[jonghoonpark]

related jira issue: https://bugs.openjdk.org/browse/JDK-8365782

---

### Changes

As described in the issue, I have removed the unnecessary inclusion of `stdlib.h`.

This was first discussed in [this comment](https://github.com/openjdk/jdk/pull/26800#discussion_r2278998210).

As noted in the discussion, `generate_environment_variables_events` also appears to be unimplemented, so it has been removed as well.

---

### Verification

- Verified that no functions from stdlib.h are called in the file.
- Checked that the `generate_environment_variables_events` function is not implemented anywhere in the codebase.
- Ran the `tier1` tests after the changes and confirmed that there were no issues.

Please let me know if there are any additional or more appropriate verification steps I should perform.

-------------

Commit messages:
 - cleanup: Remove unnecessary inclusion of <stdlib.h> in jfrOSInterface.cpp

Changes: https://git.openjdk.org/jdk/pull/27639/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27639&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365782
  Stats: 6 lines in 1 file changed: 0 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27639.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27639/head:pull/27639

PR: https://git.openjdk.org/jdk/pull/27639