Remove Traffic Ops Golang legacy tc.ApiErrorType

[rob05c]

The tc.ApiErrorType is a legacy artifact that needs removed. TO Golang has standardized around the userErr, sysErr, errCode idiom, which is more powerful and flexible, and easier to work with (no type casting). The few remaining functions in TOGo using tc.ApiErrorType need changed to use the userErr, sysErr, errCode idiom, and the symbol removed.