edition lint: unnecessary `T: 'a` annotations

[nrc]

We might want to wait until #51858 is implemented before landing (not sure), but we can begin implementation.

[nikomatsakis]

Getting this 100% right would probably be fairly hard, but we can probably start with something very simple: essentially, we just identify T: 'a where-clauses in structs and .. don't do them.

struct Foo<T: 'a> { .. }

becomes

struct Foo<T> { .. }

I tried this by hand on Rayon and it all worked great ;).