update ui/arbitrary-self-types-not-object-safe with shorter error message

Author: mikeyhew

src/test/ui/arbitrary-self-types-not-object-safe.stderr

@@ -4,15 +4,15 @@ error[E0038]: the trait `Foo` cannot be made into an object
 40 |     let x = Box::new(5usize) as Box<Foo>;
    |                                 ^^^^^^^^ the trait `Foo` cannot be made into an object
    |
-   = note: method `foo` has a non-standard `self` type. Only `&self`, `&mut self`, and `Box<Self>` are currently supported for trait objects
+   = note: method `foo` has a non-standard `self` type
 
 error[E0038]: the trait `Foo` cannot be made into an object
   --> $DIR/arbitrary-self-types-not-object-safe.rs:40:13
    |
 40 |     let x = Box::new(5usize) as Box<Foo>;
    |             ^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
    |
-   = note: method `foo` has a non-standard `self` type. Only `&self`, `&mut self`, and `Box<Self>` are currently supported for trait objects
+   = note: method `foo` has a non-standard `self` type
    = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<std::boxed::Box<Foo>>` for `std::boxed::Box<usize>`
 
 error: aborting due to 2 previous errors