Remove test for UserWarning

[getzze]

Fixes CI checks in #156

With scipy>=1.15.0 (due to commit scipy/scipy@1d57f31) no UserWarning is emitted with a Wilcoxon test with too few data.
Testing for UserWarning was not needed anyway.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.39%. Comparing base (1dd7714) to head (48a3771).
Report is 16 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
+ Coverage   91.37%   91.39%   +0.02%     
==========================================
  Files          31       34       +3     
  Lines        2493     2465      -28     
==========================================
- Hits         2278     2253      -25     
+ Misses        215      212       -3     

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[trevismd]

I think it's useful to make sure that we don't prevent scipy's warnings from being shown to the user.
However, we can't delay patching other things (e.g., #170 and #172) because of that, so I'm OK with removing this assertion, especially as it's only on older versions of py/scipy and we don't have any other occurrence that I know of.