Remove the deps on `LogDensityProblemsAD` and `Requires`

[yebai]

These dependencies can be safely removed or moved into a pkg ext to minimise the dependency footprint.

[Red-Portal]

It seems that Turing used to wrap models with LogDensityProblemsAD by default, and we need to dispatch initial_sample based on the type of the model. So I included it as a dependency for access to the types. Has something changed since?

[yebai]

We have replaced LogDensityProblemAD with DifferentiationInterface, so LogDensityProblemsAD can be removed.

[Red-Portal]

Aha okay let me deal with it