gicon: Do not mark g_icon_to_string() as to_tokens() invoker

That's not strictly true as they've different signature, and this leads
to some introspection and documentation errors.

gio/gicon.c

@@ -204,7 +204,6 @@ g_icon_to_string_tokenized (GIcon *icon, GString *s)
  * - If @icon is a #GThemedIcon with exactly one name and no fallbacks,
  *   the encoding is simply the name (such as `network-server`).
  *
- * Virtual: to_tokens
  * Returns: (nullable): An allocated NUL-terminated UTF8 string or
  * %NULL if @icon can't be serialized. Use g_free() to free.
  *