CV2-4526 remove unnecessary haystack safety switch, add simple os path exists

[DGaffney]

Description

As mentioned in the Jira ticket, the error we run into isn't with the "haystack", but instead is with the "needle", the actual file being reviewed. I think the ultimate source of the problem is intermittent presto issues (practically, today, it is likely the timeouts we're seeing with tasks just because we haven't merged the timeout fix). There's nothing we can really do to save those files, so the move here is to essentially treat that file lookup as a 404 by pre-checking if the path exists. Also gutting the previous work as its more moving parts than we need to keep.

Reference: CV2-4526

How has this been tested?

Not tested yet - should be straightforward as nothing in this is really new.

Have you considered secure coding practices when writing this code?

None

[qlty-cloud-legacy[bot]]

Code Climate has analyzed commit 90f8050f and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 80.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 82.3%.

View more on Code Climate.

[DGaffney]

In order to delete the migration, do we also need a rollback to remove the column?

I don't think anyone has actually pulled this into their local environment, nor has it gone out to any of our environments, so deleting it from the codebase should just reset us - @caiosba / @sonoransun does that sound right?

[caiosba]

@DGaffney you can confirm by checking the deployed code, I guess.