Remove `size` option from `sample_posterior_predictive`

[ricardoV94]

We should remove this option, as the call to change_rv_size will break any dependencies across resized variables.

pymc/pymc/sampling.py

Lines 1740 to 1741 in 85468b0

	if size is not None:
	vars_to_sample = [change_rv_size(v, size, expand=True) for v in vars_to_sample]