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_ast_passes/src/ast_validation.rs

@@ -1552,7 +1552,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 /// When encountering an equality constraint in a `where` clause, emit an error. If the code seems
 /// like it's setting an associated type, provide an appropriate suggestion.
 fn deny_equality_constraints(
-    this: &mut AstValidator<'_>,
+    this: &AstValidator<'_>,
     predicate: &WhereEqPredicate,
     generics: &Generics,
 ) {