VisualCollection.RemoveAt breaks when multiple null entries exist

RemoveAt takes the entry at the specified index and delegates to RemoveInternal with that item. However, if there are multiple `null` entries in the collection, RemoveInternal then only removes the first, not the one at the requested index.

This likely necessitates inlining part of what RemoveInternal does into RemoveAt, since those two approaches are a bit incompatible with each other.