Remove the _babel_filter_ file and skipBabel option from Embroider options

[mansona]

skipBabel is a build option. But in the new way of writing Ember apps, we don't want the build options to control Babel, we prefer the config file babel.config.cjs to be the source of truth. This PR removes skipBabel as a build option and removes the _babel_filter_.js whose content depends on that option.

Issue number #1962 introduces a more standard shape for babel.config.cjs. One piece of this issue is to rely on real babel config options to skip files to achieve what the build option skipBabel used to do. e.g. we could use the https://babeljs.io/docs/options#exclude option 👍 so we don't need to provide a custom escape hatch any more.

That's why the PR for the present issue should be merged only after #1962 is implemented and merged:

• First we provide a new way to use babel.config.cjs to skip files (implement the "correct solution" for babel config in vite #1962)
• Then we remove skipBabel option from Embroider (the present issue)

[mansona]

This is blocked on #2016