RFR: 8369451: Debug agent support for USE_ITERATE_THROUGH_HEAP is broken and should be removed.

[Chris Plummer]

Remove support for the debug agents USE_ITERATE_THROUGH_HEAP support, and the debugflags option used to enable it. Support has been broken for a very long time and can't possibly be relied on by anyone. Please see the CR for a full description.

Not there is a very small compatibility risk with removing the debugflags option (any script that uses it would break). But since it was only used in support of USE_ITERATE_THROUGH_HEAP, is not included in the docs, and is only included in the help output for debug builds, I think the risk is very low. If used, script failures are likely a good thing as it would call attention to the fact that the user is attempting to use functionality that doesn't (and hasn't) worked.

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Commit messages:
 - Get rid of USE_ITERATE_THROUGH_HEAP support

Changes: https://git.openjdk.org/jdk/pull/27706/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27706&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369451
  Stats: 65 lines in 3 files changed: 2 ins; 43 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/27706.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27706/head:pull/27706

PR: https://git.openjdk.org/jdk/pull/27706