RFR: 8254671: ZGC: Remove unused roots iterator types

[Per Liden]

The types ZOopStorageIterator, ZSerialOopsDo and ZParallelWeakOopsDo are no longer used and can be removed.

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Commit messages:
 - 8254671: ZGC: Remove unused roots iterator types

Changes: https://git.openjdk.java.net/jdk/pull/629/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=629&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254671
  Stats: 50 lines in 2 files changed: 0 ins; 50 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/629.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/629/head:pull/629

PR: https://git.openjdk.java.net/jdk/pull/629