RFR: 8254671: ZGC: Remove unused roots iterator types [v2]

[Per Liden]

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

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Remove includes

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/629/files
  - new: https://git.openjdk.java.net/jdk/pull/629/files/a27cf962..9612f738

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=629&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=629&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 2 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