Remove hvplot workaround in downsample_zarr notebook

[jsignell]

Based off the report in #119 (comment) there was a bug uncovered in hvplot > 0.8.4: holoviz/hvplot#1329.

For now the downsample_zarr notebook downgrades hvplot to work around the bug. And in #139 a different workaround is introduced of just passing a cartopy crs object directly.

But once the bug is resolved and a new version of hvplot is released and available in the pangeo image, we should be able to remove the workaround.