Validator: deregister attr

A attribute can be deregistered by calling SH_Validator_deregister_attr.
Note that deregistering an attr, that was never registered is considered
an error, but this may change, as technically it is not registered
afterwards and sometimes (i.e. for a blacklist) it might be preferable
to ensure, that a specific attr is not registered, but it is not clear
whether there should be an error or not.
Also the deallocating of the data used for an attr was moved to an extra
method, as this is needed in several locations and it might be subject
to change.