Remove the alt_std_name option

This option introduced in #15820 allows a custom crate to be imported in
the place of std, but with the name std. I don't think there is any
value to this. At most it is confusing users of a driver that uses this option. There are no users of
this option on github. If anyone still needs it, they can emulate it
injecting #![no_core] in addition to their own prelude.

compiler/rustc_builtin_macros/src/standard_library_imports.rs

@@ -11,7 +11,6 @@ pub fn inject(
     mut krate: ast::Crate,
     resolver: &mut dyn ResolverExpand,
     sess: &Session,
-    alt_std_name: Option<Symbol>,
 ) -> ast::Crate {
     let edition = sess.parse_sess.edition;
 
@@ -53,7 +52,7 @@ pub fn inject(
                 span,
                 ident,
                 vec![cx.attribute(cx.meta_word(span, sym::macro_use))],
-                ast::ItemKind::ExternCrate(alt_std_name),
+                ast::ItemKind::ExternCrate(None),
             ),
         );
     }