RFR: 8369451: Debug agent support for USE_ITERATE_THROUGH_HEAP is broken and should be removed [v2]

[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.
> 
> Note 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.

Chris Plummer has updated the pull request incrementally with one additional commit since the last revision:

  Get rid of no longer needed cbObjectTagInstance() function

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27706/files
  - new: https://git.openjdk.org/jdk/pull/27706/files/94429a50..d6b0e5a2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27706&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27706&range=00-01

  Stats: 36 lines in 1 file changed: 0 ins; 36 del; 0 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