Rollup merge of #137299 - nnethercote:simplify-PostOrder-customization, r=compiler-errors

Simplify `Postorder` customization.

`Postorder` has a `C: Customization<'tcx>` parameter, that gives it flexibility about how it computes successors. But in practice, there are only two `impls` of `Customization`, and one is for the unit type.

This commit simplifies things by removing the generic parameter and replacing it with an `Option`.

r? ````@saethlin````

src/tools/clippy/clippy_utils/src/mir/mod.rs

@@ -30,7 +30,7 @@ pub fn visit_local_usage(locals: &[Local], mir: &Body<'_>, location: Location) -
         locals.len()
     ];
 
-    traversal::Postorder::new(&mir.basic_blocks, location.block, ())
+    traversal::Postorder::new(&mir.basic_blocks, location.block, None)
         .collect::<Vec<_>>()
         .into_iter()
         .rev()