Remove `level`, `DataFrameLike` from `main`, `v2`

[dangotbanned]

Related

• enh: Start narwhals.stable.v2 and enforce deprecations #2814 (comment)

@dangotbanned
We can move level to v1-only can't we?

Or at least "interchange" I thought was on the chopping block?

@MarcoGorelli
yes true, i just think it'd require a larger refactor

it's not user-facing though, so not necessarily pressing?

@dangotbanned
Yeah that seems fair

How about in this PR, try removing v2.typing.DataFrameLike and adding some tests using a mock w/ __dataframe__?

I think making sure the typing on v2 rejects interchange only should be doable 🙂
We can punt the rest to another issue

Description

So now we need to start thinking on how to figure out the rest 😄