Tidy up TODOs from XIOS interface

[jwallwork23]

xios_c_interface.hpp contains three TODOs:

cxios_date cxios_date_convert_from_string(const char* str, int str_size); // TODO: unused
void cxios_calendar_wrapper_handle_create(
    xios::CCalendarWrapper** calendar_wrapper_hdl, const char* _id, int _id_len); // TODO: unused
void cxios_get_current_calendar_wrapper(xios::CCalendarWrapper** _ret); // TODO: unused

We can use the first to set the start/origin date directly from a string. The second one can be dropped as not needed. The third one is actually used so we can remove the TODO note.

[jwallwork23]

Closed by #584.