InputMethod: Resolve TODO on OS 9 where newer IBus available

[ryonakano]

Originally discussed from #2793 (comment)

Resolve the following TODO probably on OS 9 where newer IBus available:

gala/src/InputMethod.vala

Lines 262 to 265 in ea03ae4

	if (HIDDEN_TEXT in hints) {
	// TODO: Probably needs a newer version
	// ibus_hints |= IBus.InputHints.HIDDEN_TEXT;
	}