RFR: 8282620: G1/Parallel: Constify is_in_young() predicates

[Thomas Schatzl]

Hi all,

  can I have reviews for this trivial change to further constify `G1Collectedheap/ParallelScavengeHeap::is_in_young()`? Also removes `is_in_old` that does not have a definition.

Testing: local compilation, gha

Thanks,
  Thomas

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Commit messages:
 - const order (2)
 - const order
 - Initial version
 - Initial version

Changes: https://git.openjdk.java.net/jdk/pull/7679/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7679&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282620
  Stats: 5 lines in 4 files changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7679.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7679/head:pull/7679

PR: https://git.openjdk.java.net/jdk/pull/7679