commands/move: don't flatten on move; reap empty former parent instead

Some comparisons of current Sway versus i3 behavior:

1) T[T[T[app]]] + move left
* Sway: T[app]
* i3:   T[T[app]]

2) H[V[H[V[app]]]] + move left
* Sway: H[app]
* i3:   H[V[app]]

After this commit, Sway behavior matches i3. The intermediate states
are now:

    T[T[T[app]]] -> T[T[app T[]]] -> T[T[app]]
    H[V[H[V[app]]]] -> H[V[app H[V[]]]] -> H[V[app]]

sway/commands/move.c

@@ -309,16 +309,6 @@ static bool container_move_in_direction(struct sway_container *container,
 		return false;
 	}
 
-	// If container is in a split container by itself, move out of the split
-	if (container->parent) {
-		struct sway_container *old_parent = container->parent;
-		struct sway_container *new_parent =
-			container_flatten(container->parent);
-		if (new_parent != old_parent) {
-			return true;
-		}
-	}
-
 	// Look for a suitable *container* sibling or parent.
 	// The below loop stops once we hit the workspace because current->parent
 	// is NULL for the topmost containers in a workspace.
@@ -721,12 +711,20 @@ static struct cmd_results *cmd_move_in_direction(
 		return cmd_results_new(CMD_SUCCESS, NULL);
 	}
 	struct sway_workspace *old_ws = container->workspace;
+	struct sway_container *old_parent = container->parent;
 
 	if (!container_move_in_direction(container, direction)) {
 		// Container didn't move
 		return cmd_results_new(CMD_SUCCESS, NULL);
 	}
 
+	// clean-up, destroying parents if the container was the last child
+	if (old_parent) {
+		container_reap_empty(old_parent);
+	} else if (old_ws) {
+		workspace_consider_destroy(old_ws);
+	}
+
 	struct sway_workspace *new_ws = container->workspace;
 
 	if (root->fullscreen_global) {