Remove Planetary Comptuer specific code

[TomAugspurger]

I think https://github.com/TomAugspurger/stac-geoparquet/blob/main/stac_geoparquet/pc_runner.py is too Planetary Computer-specific and should be removed.

https://github.com/TomAugspurger/stac-geoparquet/blob/main/stac_geoparquet/pgstac_reader.py is a mixture of things that are generic to pgstac and Planetary Comptuer. With a small refactor, I can get a pretty good split.

xref https://github.com/TomAugspurger/stac-geoparquet/issues/14.