librustc: Remove the concept of modes from the compiler.

This commit does not remove `ty::arg`, although that should be
possible to do now.

src/libsyntax/ext/build.rs

@@ -419,7 +419,6 @@ pub fn mk_arg(cx: @ext_ctxt,
            -> ast::arg {
     let arg_pat = mk_pat_ident(cx, span, ident);
     ast::arg {
-        mode: ast::infer(cx.next_id()),
         is_mutbl: false,
         ty: ty,
         pat: arg_pat,