MAINT: remove external samplers

[mj-will]

Remove the external samplers for 3.0.

I see a few open questions:

• How do we want to handle the examples?
  • For now I've left the examples that use other samplers but added an explicit error.
• How should the tests work if external samplers are installed?
  • At the moment, they detect all available samplers, including any external samplers that are installed.
• One of the doc notebooks uses external samples, should these be installed for the entire CI or just the docs CI?
• Should we still provide sampler_requirements.txt?

[mj-will]

#1041 needs to be merged before the CI can pass.

[ColmTalbot]

• How do we want to handle the examples?
  • For now I've left the examples that use other samplers but added an explicit error.

This looks good to me, neater than what we had before.

• How should the tests work if external samplers are installed?
  • At the moment, they detect all available samplers, including any external samplers that are installed.

Probably, we should only test the ones we package.
It would be great if we can expose this test in some way that plugins can use it directly.

• One of the doc notebooks uses external samples, should these be installed for the entire CI or just the docs CI?

Yeah, I think we should have at least a few of those available, maybe in the full CI.
It might be worth changing which ones are used, restrict it to maintained, easy to install ones?

• Should we still provide sampler_requirements.txt?

I'm leaning to no, but could go either way.

[ColmTalbot]

There are probably other packages that can be removed from here, but I imagine it will be a pain to go through and figure out which.