E0091 needs to be updated to new format

[sophiajt]

From: src/test/compile-fail/E0091.rs

Error E0091 needs a span_label, updating it from:

error[E0091]: type parameter `T` is unused
  --> src/test/compile-fail/E0091.rs:11:10
   |
11 | type Foo<T> = u32; //~ ERROR E0091
   |          ^

error[E0091]: type parameter `B` is unused
  --> src/test/compile-fail/E0091.rs:12:14
   |
12 | type Foo2<A, B> = Box<A>; //~ ERROR E0091
   |              ^

To:

error[E0091]: type parameter `T` is unused
  --> src/test/compile-fail/E0091.rs:11:10
   |
11 | type Foo<T> = u32; //~ ERROR E0091
   |          ^ unused

error[E0091]: type parameter `B` is unused
  --> src/test/compile-fail/E0091.rs:12:14
   |
12 | type Foo2<A, B> = Box<A>; //~ ERROR E0091
   |              ^ unused

[stephen-lazaro]

I'll be tackling this one over the next few days.

[stephen-lazaro]

Addressed in #35528 pending merge.