Fix hue_order as a subset in scatterplot

[mwaskom]

Fixes #2419

With simpler example from that issue (as the problem didn't really have anything to do with PairGrid:

sns.scatterplot(
    data=iris,
    x="sepal_length", y="sepal_width",
    hue="species", hue_order=["setosa", "versicolor"],
)

[codecov]

Codecov Report

Merging #2848 (487dae9) into master (94621ce) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2848      +/-   ##
==========================================
- Coverage   98.05%   98.04%   -0.02%     
==========================================
  Files          69       69              
  Lines       22232    22242      +10     
==========================================
+ Hits        21800    21807       +7     
- Misses        432      435       +3     

Impacted Files	Coverage Δ
seaborn/_oldcore.py	97.66% <100.00%> (-0.41%)	⬇️
tests/test_relational.py	99.28% <100.00%> (+<0.01%)	⬆️

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