Deprecate DuckDB support in hvplot.plotting.plot

[maximlt]

The plot function in hvplot.plotting is meant to be the entry point called by Pandas directly when users set the plotting backend (via e.g. pd.options.plotting.backend = 'hvplot'. #1398 added some code in that function when DuckDB support was added, which doesn't look useful because as far as I know DuckDB doesn't implement the same plotting backend mechanism Pandas implements.

This function also includes support for Polars, which was added at the time Polars used hvPlot as its only plotting backend. Arguably this could be removed too since Polars has moved to using Altair instead, I'd keep it for a little longer.

Thoughts @ahuang11 ?

[ahuang11]

which doesn't look useful because as far as I know DuckDB doesn't implement the same plotting backend mechanism Pandas implements.

Ah, I was only coding by parallelism (following the other backends), so this is fine with me!