Redefine Debug instead of importing it

This reverts commit 7e2548fe69.

Now I know why it was redefined: it seems like it's potentially because
of the orphan rule. Here are the error messages:

error[E0119]: conflicting implementations of trait `std::fmt::Debug` for type `!`:
 --> src/primitive_docs.rs:236:1
  |
6 | impl Debug for ! {
  | ^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `core`:
          - impl std::fmt::Debug for !;

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
 --> src/primitive_docs.rs:236:1
  |
6 | impl Debug for ! {
  | ^^^^^^^^^^^^^^^-
  | |              |
  | |              `!` is not defined in the current crate
  | impl doesn't use only types from inside the current crate
  |
  = note: define and implement a trait or new type instead

library/std/src/primitive_docs.rs

@@ -233,7 +233,10 @@ mod prim_bool {}
 ///
 /// ```
 /// #![feature(never_type)]
-/// # use std::fmt::{self, Debug};
+/// # use std::fmt;
+/// # trait Debug {
+/// #     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result;
+/// # }
 /// impl Debug for ! {
 ///     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
 ///         *self