Remove unneeded threadpool code in `RelionSource`

[garrettwrong]

          I think it is actually unpacked in the future, and would be needed then , unless we keep track of the indices in another structure.

Good idea, but i'm not willing to touch that threadpool code in a rush. (it has no reason to exist, but that is another matter entirely). Will create an issue to delete it.

Originally posted by @garrettwrong in #911 (comment)

[vineetbansal]

I vaguely seem to remember the threadpool brought down the loading time of a real-life EMPIAR dataset from an hour to 15 mins, but maybe it was before the entire pipeline stuff was put in and everything was cached in memory by default. But maybe worth benchmarking.

[garrettwrong]

We can test it. Sometimes those things are artifacts of where/when they were run (example, an older machine, or slow filesystem etc). I suspect a result from one cluster 5 years ago might not be the best evidence... If it's worth having the complexity we can certainly do nothing :).

[janden]

Yes let's benchmark.

[garrettwrong]

This was removed in 1308 which was in v0.14.1. I forgot to put on the board/notes.