Drop support for Tracker

[mhauru]

Some things to remove

• Replace Tracker with Mooncake in Bayesian neural networks example docs#521
• Any other reference to Tracker in the docs
• export of AutoTracker.
• some tests still cover Tracker. Could leave them though if they are fast and harmless, maybe leave comments saying that if they ever need fixing, they should rather be deleted.
• something else? Please add.

[penelopeysm]

Replacing Tracker with Mooncake in docs is blocked, because that depends on Turing 0.35, and it's not possible right now to get a fully compiling Julia environment for the docs with Turing 0.35. (essentially, we need to bump DPPL and Turing to run with Bijectors@0.14 -- but in the process we should probably fix DPPL's CI which is a bit broken now)

TuringLang/docs#541

That said, I think the remaining issues are to do with docs, rather than the Turing library. As far as Turing.jl is concerned, this issue has been completed by #2341. Besides, I've already been keeping track of the docs stuff on the issue above. So I think it's safe to close this in favour of the docs issue.