Don't remove top left ylabel from corner pairplot with normal diagonal

[mwaskom]

Closes #2772. Because PairGrid.map_diag has logic to disable the y axis visibility when corner=True, there's no reason to remove the y axis label when setting up the plot.

[codecov]

Codecov Report

Merging #2850 (e0048e4) into master (7b3e4df) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2850      +/-   ##
==========================================
- Coverage   98.04%   98.04%   -0.01%     
==========================================
  Files          69       69              
  Lines       22242    22240       -2     
==========================================
- Hits        21807    21805       -2     
  Misses        435      435              

Impacted Files	Coverage Δ
seaborn/axisgrid.py	96.04% <ø> (-0.01%)	⬇️
tests/test_axisgrid.py	99.36% <100.00%> (ø)

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