module: subpath folder mappings EOL

This removes support for subpath folder mappings, turning the runtime deprecation into an end of life feature.

For packages that previously relied on "exports": { "./subpath/": "./subpath/" } working, these packages will now effectively either get the next best exports match or an export not defined error.

Patterns supporting trailing / matches are still supported with these cases continuing to be deprecated as per that deprecation path. This PR is distinct from https://github.com/nodejs/node/pull/40117.

Because package imports did not ship with trailing / pattern matches, that same deprecation path turns out not to be necessary for imports as that deprecation only affected one release so far.

The benefit of this deprecation is spec and implementation simplification.