Change removal notice for pointer guide.

This is a bit outdated.

src/doc/guide-pointers.md

-% The (old) Rust Pointer Guide
+% The Rust Pointer Guide
 
-This content has moved into
-[the Rust Programming Language book](book/pointers.html).
+This content has been removed, with no direct replacement. Rust only
+has two built-in pointer types now,
+[references](book/references-and-borrowing.html) and [raw
+pointers](book/raw-pointers.html). Older Rusts had many more pointer
+types, they’re gone now.