Rollup merge of #123188 - klensy:clippy-me2, r=Nilstrieb

compiler: fix few unused_peekable and needless_pass_by_ref_mut clippy lints

This fixes few instances of `unused_peekable` and `needless_pass_by_ref_mut`. While i expected to fix more warnings, `needless_pass_by_ref_mut` produced too much for one PR, so i stopped here.

Better reviewed commit by commit, as fixes splitted by chunks.

compiler/rustc_trait_selection/src/traits/util.rs

@@ -205,7 +205,7 @@ impl Iterator for SupertraitDefIds<'_> {
 /// returning the resulting subject and all obligations that arise.
 /// The obligations are closed under normalization.
 pub fn impl_subject_and_oblig<'a, 'tcx>(
-    selcx: &mut SelectionContext<'a, 'tcx>,
+    selcx: &SelectionContext<'a, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     impl_def_id: DefId,
     impl_args: GenericArgsRef<'tcx>,