Fix error E0460 when compiled on Rustc repo

Author: tesuji

tests/compile-test.rs

@@ -37,7 +37,6 @@ fn clippy_driver_path() -> PathBuf {
 //        Because it would force-rebuild if the options passed to `build` command is not the same
 //        as what we manually pass to `cargo` invocation
 fn third_party_crates() -> String {
-    use std::collections::hash_map::Entry::Vacant;
     use std::collections::HashMap;
     static CRATES: &[&str] = &["serde", "serde_derive", "regex", "clippy_lints"];
     let dep_dir = cargo::TARGET_LIB.join("deps");