Remove tests for polars.plot

[ahuang11]

Polars replaced hvplot with altair in https://github.com/pola-rs/polars/pull/17995

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.