Tweak auto trait errors

[estebank]

Make suggestions to remove params and super traits verbose.

error[E0567]: auto traits cannot have generic parameters
  --> $DIR/auto-trait-validation.rs:6:19
   |
LL | auto trait Generic<T> {}
   |            -------^^^
   |            |
   |            auto trait cannot have generic parameters
   |
help: remove the parameters
   |
LL - auto trait Generic<T> {}
LL + auto trait Generic {}
   |

error[E0568]: auto traits cannot have super traits or lifetime bounds
  --> $DIR/auto-trait-validation.rs:8:20
   |
LL | auto trait Bound : Copy {}
   |            -----   ^^^^
   |            |
   |            auto traits cannot have super traits or lifetime bounds
   |
help: remove the super traits or lifetime bounds
   |
LL - auto trait Bound : Copy {}
LL + auto trait Bound {}
   |

Make suggestion to remove associated items hidden.

error[E0380]: auto traits cannot have associated items
  --> $DIR/issue-23080.rs:5:8
   |
LL | unsafe auto trait Trait {
   |                   ----- auto traits cannot have associated items
LL |     fn method(&self) {
   |        ^^^^^^
   |
   = help: remove the associated items

Make spans smaller.

r? @compiler-errors

[compiler-errors]

I think that all of these removals are clear enough that we should probably make all of them hidden. A suggestion that says "remove the parameters" doesn't need to take 4 extra lines to render the removal of those parameters; it's pretty clear by the name and the span that's being highlighted what the user needs to do here, and really the only thing we want to make sure is that they're still suggestions so they can be applicable.

In general, I think instead of making what I'll call "unambiguous removal" suggestions verbose, it's probably best to make them hidden.