Remove RegisterOption argument registration (allow other argument types)

[gareth-cross]

While attempting to expose an argument of type std::size_t, I encountered an issue where RegisterOption rejects the argument because it is not one of bool, int, double, std::string.

I lack some context here, but as best I can tell from the code the RegisterOption call is unnecessary - the parsing is handled by boost::program_options::options_description and the values in options_int_, etc... are never accessed.

If this is the case, it is straightforward to remove the call to RegisterOption - then other argument types can be added.