RFR: 8278842: Parallel: Remove unused VerifyObjectStartArrayClosure::_old_gen

[Albert Mingkun Yang]

Trivial change of removing an unused field.

Test: build

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Commit messages:
 - trivial

Changes: https://git.openjdk.java.net/jdk/pull/6846/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6846&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278842
  Stats: 4 lines in 1 file changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6846.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6846/head:pull/6846

PR: https://git.openjdk.java.net/jdk/pull/6846