cleanup: simplify get_pbar by removing a vestigial class that's not used anymore

[neutrinoceros]

PR Summary

related to YTEP-0039 but not part of it
I noticed the parallel flag from yt.funcs.get_pbar wasn't used anywhere so I removed the unnecessary code around it.

(This will conflict with #3114. I'd like the present change to go in first)

[brittonsmith]

It is used in yt_astro_analysis. I am getting rid of it with some new changes, but maybe we can hold off on this so I don't have to fix yt-astro first.

[neutrinoceros]

I think the fix is simply to replace any instance of ParallelProgressBar with DummyProgressBar
I can either submit a patch to yt_astro_anaylsis or deprecate de class here, what do you prefer ?

[neutrinoceros]

Actually @brittonsmith , I went to yt_astro_analysis to try and it looks to mean it's not used there (anymore ?).

[munkm]

@brittonsmith let us know when this is fixed in astro_analysis and we can merge after that.

[neutrinoceros]

I've double checked and I confirm that git grep ParallelProgressBar returns nothing on the master (and only) branch there.

[neutrinoceros]

Looks like this is actually mergeable with 2 approvals ? @matthewturk @munkm ?

[brittonsmith]

For posterity, I finally remembered what package uses this. It's one of mine. I'll move the code there.