RFR: 8322204: Parallel: Remove unused _collection_cost_margin_fraction

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - pgc-remove-field

Changes: https://git.openjdk.org/jdk/pull/17124/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17124&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322204
  Stats: 3 lines in 2 files changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17124.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17124/head:pull/17124

PR: https://git.openjdk.org/jdk/pull/17124