ads.txt file detection throws a lot of false positives

[helen]

Describe the bug

We are getting a lot of reports of the admin showing the existing file notice where they don't actually have one. Many of these reports are also tied to AdSense error reports, but I have not yet investigated what that means or if they really are related or the users are noticing the notice because of the error.

Steps to Reproduce
I personally have only been able to reproduce this in a subsite on a subdirectory multisite installation that has the front end mapped to another domain (using the MU domain mapping plugin + sunrise.php). This was my original diagnosis in #48 which I think is still correct for some instances but doesn't explain all or possibly even most of the reports. My new suspicion is that there are custom 404 handlers and other things also interfering, but we need to do a lot of work communicating back and forth with reporters to really chase all these cases down to solve for them correctly at once.

Expected behavior
I expect the notice to only show if there is an actual ads.txt file that will show up instead of what you're entering in the admin, avoiding false positives as much as possible. It may also make sense to make it permanently dismissible, either on a per-user or per-site basis.

Screenshots
I can take a screenshot of the notice in question.

Environment information
Needs more info.

Additional context
IMO we need to pull this out for 1.3.0, get app-ads.txt out there due to increased demand, and work on a more robust and intelligent solution. So I would like to see a PR reverting #19, and then we can work on a separate PR to actually bring back the notice in a sane way (and add it for app-ads.txt as well).