Fix warning about let_underscore_drop lint removal from clippy

warning: lint `clippy::let_underscore_drop` has been renamed to `let_underscore_drop`
      --> gen/build/src/lib.rs:59:5
       |
    59 |     clippy::let_underscore_drop,
       |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `let_underscore_drop`
       |
       = note: `#[warn(renamed_and_removed_lints)]` on by default

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

    warning: lint `clippy::let_underscore_drop` has been renamed to `let_underscore_drop`
     --> tests/unique_ptr.rs:1:10
      |
    1 | #![allow(clippy::let_underscore_drop)]
      |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `let_underscore_drop`
      |
      = note: `#[warn(renamed_and_removed_lints)]` on by default

gen/build/src/lib.rs

@@ -56,7 +56,6 @@
     clippy::if_same_then_else,
     clippy::inherent_to_string,
     clippy::items_after_statements,
-    clippy::let_underscore_drop,
     clippy::match_bool,
     clippy::match_on_vec_items,
     clippy::match_same_arms,

tests/ffi/lib.rs

@@ -2,7 +2,6 @@
     clippy::boxed_local,
     clippy::derive_partial_eq_without_eq,
     clippy::just_underscores_and_digits,
-    clippy::let_underscore_drop,
     clippy::missing_safety_doc,
     clippy::must_use_candidate,
     clippy::needless_lifetimes,

tests/unique_ptr.rs

@@ -1,5 +1,3 @@
-#![allow(clippy::let_underscore_drop)]
-
 use cxx::{CxxString, UniquePtr};
 
 #[test]