Migrate animated_hillshade example script off datashader

[brendancol]

Reason or Problem

examples/animated_hillshade.py uses datashader only for rendering (shade, stack, the Elevation colormap). There is no rasterization to replace here.

Proposal

• Remove the datashader imports.
• Replace shade/stack rendering with matplotlib, or, if the script is stale and no longer referenced, remove it instead.
• Confirm the script still runs (or document its removal).

Done when

• The script runs with no datashader import, or it is removed with a note on why.

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