drop `min_confidence`

[dpprdan]

OpenCage (the API) has dropped support for min_confidence (the parameter kind of still works, but not always reliably, e.g. #161).

I am leaning towards removing all code w.r.t. min_confidence and to just issue a warning that the parameter is not supported anymore. There is not much use passing it through to the API if the results aren't reliable. Also it apparently hasn't been used much in practice and was often misunderstood anyways.