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_mir_transform/src/remove_unneeded_drops.rs

@@ -4,7 +4,6 @@
 //! useful because (unlike MIR building) it runs after type checking, so it can make use of
 //! `Reveal::All` to provide more precise type information.
 
-use crate::MirPass;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;