remove extra imports from rustc

Author: DevinR528

clippy_lints/src/lib.rs

@@ -58,6 +58,7 @@ extern crate rustc_trait_selection;
 #[allow(unused_extern_crates)]
 extern crate rustc_typeck;
 
+use rustc::session::Session;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_lint::LintId;
 use rustc_session::Session;