RFR: 8322543: Parallel: Remove unused _major_pause_old_slope_counter

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17168/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17168&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322543
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17168.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17168/head:pull/17168

PR: https://git.openjdk.org/jdk/pull/17168