near-vm: Make rayon unconditional and remove `into_par_iter_if_rayon`

[nagisa]

Describe the bug

Making use of rayon optional is causing us to take suboptimal implementation decisions that hurt both rayon and non-rayon use-cases (such as collecting twice, first into a vector, then into PrimaryMap.)

If making use of rayon unconditional (if you need to limit worker threads, setup the thread pool accordingly) makes way for simplifying those areas of code, we should just go ahead with it.

[matklad]

My gut feeling is that we should unconditionally not use rayon perhaps? It won't help with worst-case, and, architecture-wise, it is perhaps more prudent to add || on a higher level:

• when compiling a bunch of contracts, compile them in parallel (catchup mode) (already doing this)
• when compiling a single contract during deploy, compile it off the main thread. (not doing this yet, but could do)

[nagisa]

That would work too, sure.

[matklad]

Ok, let's just remove rayon then!

[Ekleog-NEAR]

Related discussion: https://github.com/near/nearcore-private-1/issues/2#issuecomment-1256030324