Remove vars from sample_posterior_predictive, and other small refactorings

[MarcoGorelli]

No description provided.

[codecov]

Codecov Report

Merging #4343 (ca0c28b) into master (9dbf9ea) will increase coverage by 0.01%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master    #4343      +/-   ##
==========================================
+ Coverage   87.85%   87.86%   +0.01%     
==========================================
  Files          88       88              
  Lines       14501    14467      -34     
==========================================
- Hits        12740    12712      -28     
+ Misses       1761     1755       -6     

Impacted Files	Coverage Δ
pymc3/sampling.py	89.62% <55.55%> (-0.08%)	⬇️
pymc3/distributions/dist_math.py	91.85% <0.00%> (-0.12%)	⬇️
pymc3/distributions/__init__.py	100.00% <0.00%> (ø)
pymc3/distributions/discrete.py	96.78% <0.00%> (+1.56%)	⬆️