Clean up tests/ui by removing `does-nothing.rs`
In [a previous PR](https://github.com/rust-lang/rust/pull/123297#issuecomment-2039887806), it was suggested that this test be removed:
> it's testing a basic diagnostic for an unknown variable (added over a decade ago for https://github.com/rust-lang/rust/issues/154) that is already covered by probably dozens or hundreds of other tests.
It was then suggested that [opening a new PR](https://github.com/rust-lang/rust/pull/123563#discussion_r1554654102) for this would be more organized.
I'm setting this as a draft, as:
1. The tests/ui directory is rather disorganized, a large quantity of tests are not even contained inside their own directories. This PR could turn into "clean up the UI tests directory", if I were to place everything into categories (for example, everything related to CLI flags could get placed in a cli directory).
2. This will have a merge conflict with #123563 should that get merged. I trust that _this time_, I won't run into [The Incident](https://github.com/rust-lang/rust/pull/123297#issuecomment-2041137569) while rebasing. Edit: Yay, I did it properly!
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