Fix renamed let_underscore_drop lint

    warning: lint `clippy::let_underscore_drop` has been renamed to `let_underscore_drop`
     --> tests/test.rs:3:5
      |
    3 |     clippy::let_underscore_drop,
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `let_underscore_drop`
      |
      = note: `#[warn(renamed_and_removed_lints)]` on by default

Author: dtolnay

tests/test.rs

@@ -1,6 +1,5 @@
 #![allow(
     clippy::disallowed_names,
-    clippy::let_underscore_drop,
     clippy::shadow_unrelated,
     clippy::unseparated_literal_suffix,
     clippy::used_underscore_binding