RFR: 8279510: Parallel: Remove unused PSScavenge::_consecutive_skipped_scavenges

[Albert Mingkun Yang]

Simple change of removing effectively unused code.

Test: hotspot_gc

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Commit messages:
 - consecutive_skipped_scavenges

Changes: https://git.openjdk.java.net/jdk/pull/6965/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6965&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279510
  Stats: 12 lines in 2 files changed: 0 ins; 11 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6965.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6965/head:pull/6965

PR: https://git.openjdk.java.net/jdk/pull/6965