Fix renamed let_underscore_drop lint

    warning: lint `clippy::let_underscore_drop` has been renamed to `let_underscore_drop`
     --> tests/test_ensure.rs:8:5
      |
    8 |     clippy::let_underscore_drop,
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `let_underscore_drop`
      |
      = note: `#[warn(renamed_and_removed_lints)]` on by default

tests/test_ensure.rs

@@ -5,7 +5,6 @@
     clippy::ifs_same_cond,
     clippy::items_after_statements,
     clippy::let_and_return,
-    clippy::let_underscore_drop,
     clippy::match_bool,
     clippy::never_loop,
     clippy::overly_complex_bool_expr,