Migrate classification-methods example off datashader

[brendancol]

Reason or Problem

examples/xarray-spatial_classification-methods.ipynb uses datashader only for rendering (shade with matplotlib colormaps). There is no rasterization to replace here.

Proposal

• Remove the datashader import.
• Replace shade with matplotlib imshow using the same colormaps.
• Update any prose that mentions datashader.
• Re-execute the notebook so outputs regenerate cleanly.

Done when

• The notebook runs top to bottom with no datashader import and produces the expected plots.

Part of removing datashader as a dependency; final packaging removal is tracked in #3450.