Remove parallel iterators in attribute processors

[teh-cmc]

At least on my machine, the parallel iterators that were recently added to the attribute processing methods result in at least an order of magnitude performance loss.

With par_iter:

Without par_iter:

(Screenshots taken from #5000)

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[emilk]

Oh no, these we're very helpful for #4991 but, I guess I'll cross that bridge when I get back to it. Maybe with a "if large, use rayon, else don't" 🙄

It really makes me wish we had benchmarks for this though 😭