Fix renamed let_underscore_drop lint

    error: lint `clippy::let_underscore_drop` has been renamed to `let_underscore_drop`
     --> tests/test.rs:9:5
      |
    9 |     clippy::let_underscore_drop,
      |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use the new name: `let_underscore_drop`
      |
      = note: `-D renamed-and-removed-lints` implied by `-D warnings`

Author: dtolnay

tests/test.rs

@@ -6,7 +6,6 @@
     clippy::excessive_precision,
     clippy::float_cmp,
     clippy::items_after_statements,
-    clippy::let_underscore_drop,
     clippy::shadow_unrelated,
     clippy::too_many_lines,
     clippy::unreadable_literal,