Remove`units.pyi` file

[IAlibay]

As per today's discussion with @mattwthompson

It isn't clear to us that units.pyi is still required for typing. At least running mypy downstream doesn't seem to require it anymore.

[mattwthompson]

Another quick note: given how many downstream packages use this and want type-checking to work, we should put more work into downstream_dummy to handle common use cases