ENH: Remove features that were deprecated in ITKv5

[hjmjohnson]

Deprecated capabilities of ITKv5 are now removed
in ITKv6.

Deprecated API's removed.

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)

[thewtex]

🚀 👍

Thanks, @hjmjohnson !

Please add an entry in the Migration Guide.

[dzenanz]

A short entry in migration guide should suffice. E.g.:

All contents of the Deprecated module were removed. This includes TreeContainer and related classes; atomic primitives, mutexes and related classes which are now part of C++ standard; specialized Vector filters - specialized versions are no longer needed, as regular filters can work with vector images. For details, see https://github.com/InsightSoftwareConsortium/ITK/blob/master/Documentation/docs/migration_guides/itk_5_migration_guide.md

[dzenanz]

##[error]This is a scheduled macOS-11 brownout. The macOS-11 environment is deprecated and will be removed on June 28th, 2024.

[hjmjohnson]

##[error]This is a scheduled macOS-11 brownout. The macOS-11 environment is deprecated and will be removed on June 28th, 2024.

That error is related to: #4726
which is related to updating the dashboard needed to find the failure.

[hjmjohnson]

/azp run ITK.macOS

[hjmjohnson]

/azp run ITK.macOS.Python