Audit uses of `FnCtxt::impl_self_ty`

[Centril]

Context: #69340 (comment)

cc @eddyb

[eddyb]

IMO it should be possible to fully remove (see comment linked by @Centril).

If it was more widely used (or if removing it doesn't seem to work), it would be important to rename to have instantiate in the name to indicate it introduces inference variables.

[JohnTitor]

I just tried removing impl_self_ty fully, it seems to work well: https://github.com/JohnTitor/rust/tree/impl-self-ty

Could I submit this patch?

[Centril]

cc also @nikomatsakis ^---
Error messages are improved at least. :)

[eddyb]

@JohnTitor In suggestions you probably don't need normalize_ty either, AFAICT the type is just used to describe the impl to the user.

[JohnTitor]

@eddyb You're right, thanks! I'd like to submit a PR if you could do further reviews :)