Remove tests for polars.plot

[ahuang11]

Polars replaced hvplot with altair in pola-rs/polars#17995

FAILED hvplot/tests/plotting/testcore.py::test_series_polars_downstream[violin] - polars.exceptions.ModuleUpgradeRequiredError: altair>=5.4.0 is required for .plot

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.80%. Comparing base (6c96c7e) to head (1424f78).
Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1403      +/-   ##
==========================================
+ Coverage   87.39%   88.80%   +1.40%     
==========================================
  Files          50       51       +1     
  Lines        7490     7494       +4     
==========================================
+ Hits         6546     6655     +109     
+ Misses        944      839     -105     

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