Time to remove polychord?

After reworking the test suite, we're still having issues with installing polychord because it has to be built from source.

I suggest that we begin the process of removing polychord as a supported sampler. With the following process:

• remove it from the test suite ASAP.
• remove it from the package at the next major/minor release (presumably 2.3).

This would be the first time we've dropped support for a sampler, but I think it would be good to formalize that any supported sampler should be available from conda-forge.

Thoughts @gregory.ashton @michael.williams @sylvia.biscoveanu?