Remove Sized on_unimplemented note

src/test/ui/error-codes/E0277.stderr

@@ -2,10 +2,9 @@ error[E0277]: the size for values of type `[u8]` cannot be known at compilation
   --> $DIR/E0277.rs:13:6
    |
 LL | fn f(p: Path) { }
-   |      ^ borrow the `Path` instead
+   |      ^ doesn't have a size known at compile-time
    |
    = help: within `std::path::Path`, the trait `std::marker::Sized` is not implemented for `[u8]`
-   = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = note: required because it appears within the type `std::path::Path`
    = note: all local variables must have a statically known size
    = help: unsized locals are gated as an unstable feature