Wrong logp function in ElemwiseCategoricalStep

[junpenglao]

cc @twiecki, related #1563

It seems that the logp being evaluated in ElemwiseCategoricalStep is incorrect:
ElemwiseCategorical evaluate the logp of the RV and its parent:
https://github.com/pymc-devs/pymc3/blob/fde52a4a69be1b0887a2f7861801fb48c941bbe6/pymc3/step_methods/gibbs.py#L39-L40
https://github.com/pymc-devs/pymc3/blob/fde52a4a69be1b0887a2f7861801fb48c941bbe6/pymc3/step_methods/gibbs.py#L53-L56

Instead, it should evaluate on the conditional logp only (as in Gibbs sampling).

[junpenglao]

cc @rrkarim

[ricardoV94]

Method was removed in #4701