Remove the duplicate exercise list screen and dead reorder stubs

ExerciseListView was a near-duplicate of RoutineDetailView's exercise
management, reachable only from RoutineAddEditView's "Exercises" row —
drop both, leaving RoutineDetailView as the single exercise manager.
OrderableItem and SortableForEach were self-declared dead stubs.

Claude-Session: https://claude.ai/code/session_01H8VxUX4ckjU3vRF5M4L5FV

Workouts/Views/Routines/RoutineAddEditView.swift

@@ -160,18 +160,7 @@ struct RoutineAddEditView: View {
                 }
             }
 
-            if let routine = routine {
-                Section(header: Text("Exercises")) {
-                    NavigationLink {
-                        ExerciseListView(routine: routine)
-                    } label: {
-                        ListItem(
-                            text: "Exercises",
-                            count: routine.exercisesArray.count
-                        )
-                    }
-                }
-
+            if routine != nil {
                 Section {
                     Button("Delete Routine", role: .destructive) {
                         showingDeleteConfirmation = true