[docs] remove confusing assignment from stores tutorial (#6867)

Tutorial on writable stores contains unexplained and unused assignment
which might cause confusion.

Proper explanations on `unsubscribe` is in the next step of the tutorial.

site/content/tutorial/08-stores/01-writable-stores/app-b/App.svelte

@@ -6,7 +6,7 @@
 
 	let count_value;
 
-	const unsubscribe = count.subscribe(value => {
+	count.subscribe(value => {
 		count_value = value;
 	});
 </script>