RFR: 8310146: Removing unused PerfLongVariant::_sampled

[Albert Mingkun Yang]

Trivial removing unused field.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/14493/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14493&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310146
  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14493.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14493/head:pull/14493

PR: https://git.openjdk.org/jdk/pull/14493