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/elaborate_drops.rs

@@ -1,5 +1,4 @@
 use crate::deref_separator::deref_finder;
-use crate::MirPass;
 use rustc_index::bit_set::BitSet;
 use rustc_index::IndexVec;
 use rustc_middle::mir::patch::MirPatch;