lint: port unused import braces diagnostics

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_error_messages/locales/en-US/lint.ftl

@@ -281,3 +281,5 @@ lint-path-statement-no-effect = path statement with no effect
 
 lint-unused-delim = unnecessary {$delim} around {$item}
     .suggestion = remove these {$delim}
+
+lint-unused-import-braces = braces around {$node} is unnecessary

compiler/rustc_lint/src/unused.rs

@@ -1118,7 +1118,7 @@ fn check_use_tree(&self, cx: &EarlyContext<'_>, use_tree: &ast::UseTree, item: &
             };
 
             cx.struct_span_lint(UNUSED_IMPORT_BRACES, item.span, |lint| {
-                lint.build(&format!("braces around {} is unnecessary", node_name)).emit();
+                lint.build(fluent::lint::unused_import_braces).set_arg("node", node_name).emit();
             });
         }
     }