update: recursively purge deleted directories

When a directory is deleted, MPD deleted only the directory from the
database; it did not bother to walk the full tree to free all memory
and to remove deleted songs from the playlist.  Replace a
dirvec_delete() with delete_directory().

src/update.c

@@ -225,7 +225,7 @@ removeDeletedFromDirectory(struct directory *directory)
 			continue;
 
 		g_debug("removing directory: %s", dv->base[i]->path);
-		dirvec_delete(dv, dv->base[i]);
+		delete_directory(dv->base[i]);
 		modified = true;
 	}