Refactor: target's setRequired must be removed and replaced by 0, 1 settings

Need more research. Looks like target's `setRequire` (also `isRequired`) rare used and can be removed. It's outdated and weird logic to make target choose dialog a cancellable like cost payment dialog. Used for AI and human players in some choose target dialogs.

Actual logic for optional targets -- set 0, 1 to num targets. No needs in cancel buttons at all.