Remove getting local HIT id's from Participants table

[evankirkiles]

Speeds up querying local HIT id's vastly, especially on AWS accounts with many participants. For example, my dashboard loaded the local HITs in about 8 seconds before this commit, and in <1 after this commit.

Doesn't seem like there would ever be a case in which Participants would have a HIT Id not in the HIT Id table anyways, unless you're talking about really old backwards compatibility to before the HIT table would be a thing.

[coveralls]

Coverage increased (+0.2%) to 59.993% when pulling 5d0d6ae on evankirkiles:evan-queryspeedfixup into e96ae64 on NYUCCL:master.

[deargle]

Actually yeah, I remembered, needing the hitid search to work for people upgrading was the rationale for that check of the participants table. I'm nervous -- even though I added that functionality to master long ago, it didn't get into the pip release until like a month ago with the release of psiturk 3.

I think we need some kind of db migrate script? Something like psiturk migrate-db that, in this case, would copy hitids over from the participants table into the hits table. I would call the command something ambitious like psiturk upgrade, but I'm not ready to think about the logic of updating users' config.txt files etc.

[deargle]

Thanks @evankirkiles ! I'm not sure how to best document the need for legacy experiments to run psiturk migrate db. I guess I'll just wait for panicked support requests :-(