std: Remove format_strbuf!()

This was only ever a transitionary macro.

src/libsyntax/ext/format.rs

@@ -895,8 +895,8 @@ pub fn expand_preparsed_format_args(ecx: &mut ExtCtxt, sp: Span,
     match parser.errors.shift() {
         Some(error) => {
             cx.ecx.span_err(efmt.span,
-                            format_strbuf!("invalid format string: {}",
-                                           error).as_slice());
+                            format!("invalid format string: {}",
+                                    error).as_slice());
             return DummyResult::raw_expr(sp);
         }
         None => {}