Retire the legacy shell and PowerShell entry points #310

The pixi/Python build system (#309) supersedes the shell and PowerShell
entry points, but they cannot be removed yet: remote modules fetch three
of them by URL at a pinned tag, so deleting them breaks those consumers
until each one is updated. #309 marks them "backward compatibility" in
the README; this issue tracks actually retiring them.

Not urgent. Nothing is broken today. This is cleanup to schedule once
the consumer workflows move.

What is left

Ten files, 1,469 lines, in three tiers.

Tier 1 — already dead, no consumer (313 lines)

Referenced only by each other and the .rst docs. docs/make.bat is the
Sphinx Windows helper and has no caller at all.

Tier 2 — internal plumbing, reachable only through tier 3 (602 lines)

dockcross-manylinux-build-module-wheels.sh (219),
dockcross-manylinux-build-wheels.sh (148),
dockcross-manylinux-download-cache.sh (135),
docker_build_environment_driver.sh (100).

Tier 3 — the published API (554 lines)

These are fetched from raw.githubusercontent.com at a pinned tag.
Removing them is the compatibility break.

Why the break is smaller than it looks

Remote modules do not curl these directly. They call the reusable
workflow in
ITKRemoteModuleBuildTestPackageAction,
and the curl happens there. So the lockstep change is one file in one
repository, and modules tracking @main pick it up on their next run.
Modules pinned to an older tag keep fetching the old scripts from that
tag, which still resolves.

Suggested order

1. Update the reusable workflow to call build_wheels.py directly.
2. Confirm a real remote module builds on all four platforms through the
   updated workflow.
3. Delete tier 3, then tier 2 (unreachable once tier 3 is gone).
4. Delete tier 1 independently; nothing depends on it.

Tier 1 could be done at any time, including before the rest.

Related

• WIP: ENH: Replace the shell wheel build system with Python (supersedes #302) #309 — the build system that supersedes these