Add is_unit_expr to deprecated lints list

clippy_lints/src/lib.rs

@@ -247,6 +247,10 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         "string_to_string",
         "using `string::to_string` is common even today and specialization will likely happen soon",
     );
+    store.register_removed(
+        "is_unit_expr",
+        "superseded by `let_unit_value` and `unit_arg`",
+    );
     // end deprecated lints, do not remove this comment, it’s used in `update_lints`
 
     reg.register_late_lint_pass(box serde_api::Serde);