Use String::new() instead of "".to_string() part 2

Author: jackh726

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -2969,7 +2969,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 return Some((
                                     vec![
                                         (box_new.span.to(l_paren), String::new()),
-                                        (r_paren, "".to_string()),
+                                        (r_paren, String::new()),
                                     ],
                                     "consider removing the Box".to_string(),
                                     Applicability::MachineApplicable,