RFR: 8260643: Remove parallel version handling in CardTableRS::younger_refs_in_space_iterate()

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that removes parallel handling in `CardTableRS::younger_refs_in_space_iterate` as it is always called with n_threads <= 1, making the parallel code handling there obsolete. 

A larger cleanup of `CardTableRS` will follow in JDK-8234534.

Testing:
tier1,2

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

Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/2333/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2333&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260643
  Stats: 103 lines in 7 files changed: 3 ins; 72 del; 28 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2333.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2333/head:pull/2333

PR: https://git.openjdk.java.net/jdk/pull/2333