Group element hash generation restricts range unnecessarily

[AaronFeickert]

The hash-to-group function GroupElement::generate performs a conditional negation that restricts the output range unnecessarily: https://github.com/firoorg/firo/blob/master/src/secp256k1/src/cpp/GroupElement.cpp#L383-L385

This function appears only to be used for generating public parameters, but should be documented as unsafe for future use.