RFR: 8279523: Parallel: Remove unnecessary PSScavenge::_to_space_top_before_gc

[Albert Mingkun Yang]

Simple change of removing an unnecessary field.

Test: hotspot_gc

-------------

Commit messages:
 - save_to_space_top_before_gc

Changes: https://git.openjdk.java.net/jdk/pull/6968/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6968&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279523
  Stats: 16 lines in 3 files changed: 0 ins; 15 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6968.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6968/head:pull/6968

PR: https://git.openjdk.java.net/jdk/pull/6968