`del "del"`: remove `del` flag

[penelopeysm]

Once InitContext is fully merged, SampleFrom{Prior,Uniform}, as well as setval_and_resample!, will have been removed. This means that the only code left that uses the del flag is upstream in Turing's pMCMC samplers.

Following TuringLang/Turing.jl#2629, it is clear that there is no need for a per-varname del flag on VarInfo. A single VarInfo-wide boolean, del flag would suffice. Thus at the very least this simplification should be implemented.

Going beyond that, we could potentially manage this using contexts instead of a del flag. That will require some extra investigation.

Note that this post has been edited based on upstream changes in Turing.

[mhauru]

Would definitely want to get rid of "del" at the same time as num_produce and all the other stuff only needed by PG. Maybe this should be considered as part of v0.37?

[penelopeysm]

Happy to either lump both into 0.37, or keep both out.

(This does seem like a good candidate for sitting in the office to do it)

[penelopeysm]

closed #1058 #1060