Remove an outdated test

This... is a weird test.

It has two impls:
- `impl<T> From<Foo<T>> for Box<T>` (commented out, more on that later), and
- `impl<T> Into<Vec<T>> for Foo<T>`

The idea of that test is to show that the first impl doesn't compile, but
the second does, thus `TryFrom` should be using `Into` and not `From`
(because `Into` is more general, since the `From` impl doesn't compile).

However:
1. The types are different -- `Box` vs `Vec`, which is significant b/c
   `Box` is fundamental
2. The commented out impl actually compiles! (which wasn't detected b/c
   it's commented out :\ )

Here is a table for compilation of the impls:

|        | `Vec`        | `Box`          |
|--------|--------------|----------------|
| `From` | since 1.41.0 | never          |
| `Into` | always       | not since 1.28 |

[godbolt used to test this](https://godbolt.org/z/T38E3jGKa)

Order of events:
1. in `1.28` the `incoherent_fundamental_impls` lint becomes deny by
   default (this is *not* mentioned in the changelog yay)
2. `1.32` changed absolutely nothing, even though this version is credited
   in the test
3. the test was added (I'm not exactly sure when)
   (see https://github.com/rust-lang/rust/pull/56796)
4. in `1.41` coherence was relaxed to allow `From`+`Vec` to compile

To conclude: since `1.41` this test does nothing (and before that it was
written in a way which did not detect this change). It looks to me like
today (since `1.41`) we *could* bound `TryFrom` impl with `From` (but now
it'd be a useless breaking change of course).

Am I missing anything? Is there a useful version of this test that could
be written?