Removing converters from SampleChains

SampleChains.jl and SampleChainsDynamicHMC.jl haven't been updated in 3 years, and at multiple times we've had to work around them being incompatible with the latest versions of our other test/docs dependencies. I think it's time to remove `from_samplechains`. If maintenance on these packages resumes, we can always add back the functionality, or the converter can be relocated to the Soss/Tilde ecosystem.