Remove special-cased stacked diagonal histograms in PairGrid

[mwaskom]

Also changes the pairplot defaults:

sns.pairplot(iris, hue="species")

[codecov]

Codecov Report

Merging #1464 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1464      +/-   ##
==========================================
+ Coverage   94.42%   94.43%   +<.01%     
==========================================
  Files          15       15              
  Lines        4324     4310      -14     
==========================================
- Hits         4083     4070      -13     
+ Misses        241      240       -1

Impacted Files	Coverage Δ
seaborn/axisgrid.py	96.54% <100%> (+0.19%)	⬆️
seaborn/utils.py	86.97% <0%> (-0.39%)	⬇️

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