remove redundant imports

detects redundant imports that can be eliminated.

for #117772 :

In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.

compiler/rustc_macros/src/lift.rs

@@ -1,5 +1,5 @@
 use quote::quote;
-use syn::{self, parse_quote};
+use syn::parse_quote;
 
 pub fn lift_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::TokenStream {
     s.add_bounds(synstructure::AddBounds::Generics);