Remove direct grpcio dependency

[cretz]

What was changed

Removed the direct grpcio dependency and rely on PyPI version still.

Why?

In issue #46, we tried to mitigate the lack of grpcio mac arm support by referencing a dependency directly. While this is ok during build time, it is not when uploading to PyPI:

Invalid value for requires_dist. Error: Can't have direct dependency: 'grpcio @ https://files.pythonhosted.org/packages/61/dd/d8eda79197a1275e06621850775c7bbf34a141ff92553754dde4e87d7551/grpcio-1.46.3.tar.gz ; sys_platform == "darwin" and platform_machine

So we have removed it. Preliminary tests show this may end up building from source for grpcio when pip gets it as a dependency. This is the best we can do right now.