syntax: revert ast::AsyncArgument and associated changes.

Here follows the main reverts applied in order to make this commit:

Revert "Rollup merge of #60676 - davidtwco:issue-60674, r=cramertj"

This reverts commit 45b09453db, reversing
changes made to f6df1f6c30.

Revert "Rollup merge of #60437 - davidtwco:issue-60236, r=nikomatsakis"

This reverts commit 16939a50ea, reversing
changes made to 12bf981552.

Revert "Rollup merge of #59823 - davidtwco:issue-54716, r=cramertj"

This reverts commit 62d1574876, reversing
changes made to 4eff8526a7.

src/libsyntax_ext/deriving/debug.rs

@@ -129,7 +129,6 @@ fn stmt_let_undescore(cx: &mut ExtCtxt<'_>, sp: Span, expr: P<ast::Expr>) -> ast
         id: ast::DUMMY_NODE_ID,
         span: sp,
         attrs: ThinVec::new(),
-        source: ast::LocalSource::Normal,
     });
     ast::Stmt {
         id: ast::DUMMY_NODE_ID,