Remove unused extern crates

clippy_lints/src/lib.rs

@@ -16,14 +16,9 @@
 extern crate syntax_pos;
 #[macro_use]
 extern crate rustc;
-extern crate rustc_data_structures;
 
 extern crate toml;
 
-// Only for the compile time checking of paths
-extern crate core;
-extern crate alloc;
-
 // for unicode nfc normalization
 extern crate unicode_normalization;