Fix diagnostics to invalid metavar exprs

Suggesting to remove the entire thing makes no sense, especially not as
MachineApplicable.

Author: Xiretza

compiler/rustc_expand/src/mbe/metavar_expr.rs

@@ -98,11 +98,8 @@ impl MetaVarExpr {
             _ => {
                 let err_msg = "unrecognized meta-variable expression";
                 let mut err = psess.dcx().struct_span_err(ident.span, err_msg);
-                err.span_suggestion(
-                    ident.span,
+                err.help(
                     "supported expressions are count, ignore, index and len",
-                    "",
-                    Applicability::MachineApplicable,
                 );
                 return Err(err);
             }

tests/ui/macros/rfc-3086-metavar-expr/syntax-errors.stderr

@@ -188,7 +188,9 @@ error: unrecognized meta-variable expression
   --> $DIR/syntax-errors.rs:140:33
    |
 LL |     ( $( $i:ident ),* ) => { ${ aaaaaaaaaaaaaa(i) } };
-   |                                 ^^^^^^^^^^^^^^ help: supported expressions are count, ignore, index and len
+   |                                 ^^^^^^^^^^^^^^
+   |
+   = help: supported expressions are count, ignore, index and len
 
 error: expected identifier or string literal
   --> $DIR/syntax-errors.rs:118:33