Remove canvas_like and datashader references from source code

[brendancol]

Reason or Problem

xrspatial/utils.py defines canvas_like(), which wraps datashader.Canvas.raster() to resample a raster. It is the only place in the library source that imports datashader, it is not exported from __init__.py, and a repo-wide search finds no callers. A couple of docstrings and a comment also mention datashader.

Proposal

• Delete canvas_like() from xrspatial/utils.py, including its lazy import datashader as ds and the try/except that raises ImportError. Remove any imports the deletion leaves unused.
• Rewrite the suggest_zonal_canvas() docstring example in xrspatial/zonal.py to use arr.xrs.rasterize(...) instead of ds.Canvas().polygons(...). The function body has no datashader dependency, so this is a docstring-only change.
• Reword the comment at xrspatial/terrain.py:698 so it describes the pixel-center coordinate convention without naming datashader.

Value

Removes the library's only datashader import and trims stale references, a prerequisite for dropping datashader from packaging.

Done when

• pytest xrspatial/tests/ passes.
• No test or module imports canvas_like.

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