RFR: 8357406: Remove usages of jdk.tracePinnedThreads system property from httpclient tests

[Jaikiran Pai]

Can I please get a review of this test-only change which removes the usage of the outdated `jdk.tracePinnedThreads` system property from the httpclient tests? This addresses https://bugs.openjdk.org/browse/JDK-8357406.

As noted in the linked issue, this system property is no longer supported by the Virtual Threads implementation in the JDK.

These tests continue to pass after this change.

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Commit messages:
 - 8357406: Remove usages of jdk.tracePinnedThreads system property from httpclient tests

Changes: https://git.openjdk.org/jdk/pull/25347/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25347&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357406
  Stats: 30 lines in 7 files changed: 0 ins; 14 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/25347.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25347/head:pull/25347

PR: https://git.openjdk.org/jdk/pull/25347