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_incremental/src/persist/dirty_clean.rs

@@ -32,7 +32,6 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::TyCtxt;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::Span;
-use std::iter::FromIterator;
 use thin_vec::ThinVec;
 
 const LOADED_FROM_DISK: Symbol = sym::loaded_from_disk;