Harmonize overlapping AAMVA country code sets

[cdmgtri]

There are two AAMVA country code sets supporting the DHS Western Hemisphere Travel Initiative (WHTI) for the Enhanced Driver License (EDL):

• aamva_d20:CountryCodeType
• aamva_d20:DHSDriverLicenseIssuingCountryCodeType

Both code sets share the same codes:

Code	Definition
CAN	Canada
USA	U.S.A.

Recommendations:

• Harmonize the duplicate code sets into a single aamva_d20:CountryWHTICodeType
• Update the type of property j:CountryWHTICode to the new type.
• Remove property j:DriverLicenseIssuingCountryCode since it will no longer be necessary with the full set of NIEM country representations available via nc:CountryType. See niemopen/niem-model#22 for more.

[cdmgtri]

Pending final approval from AAMVA. Holding for 6.1.