Use collection.length = lastIndex instead of pop() in swap-and-pop

Skips the unused return-value read from pop(). Semantically identical
(both truncate the array by one; the packed-array fast path is
preserved in either form). Also byte-identical to the same change in
NVP's upstream #21221, simplifying side-by-side review.

Author: johanrd

packages/@glimmer/destroyable/index.ts

@@ -71,7 +71,7 @@ function remove<T extends object>(collection: OneOrMany<T>, item: T, message: st
     if (index !== lastIndex) {
       collection[index] = collection[lastIndex] as T;
     }
-    collection.pop();
+    collection.length = lastIndex;
     return collection;
   } else {
     return null;