Remove `NoDist` in favour of `fix`.

[yebai]

DynamicPPL.jl/src/distribution_wrappers.jl

Lines 32 to 35 in d0cfaaf

	struct NoDist{variate,support,Td<:Distribution{variate,support}} <:
	Distribution{variate,support}
	dist::Td
	end

can be removed without regression now that we have fix(model, (var=...)). The semantics of NoDist and fix are identical, while fix is more compositional and avoids hardcoding fixed variable information in model specification.