Remove unused google_web_risk_scan.py script

The PR that added this script (#244)
did so in order to fix an issue (#214)
that requested a _one-off_ check of Checkmate's allow-list against
Google Web Risk. Since this was only ever intended to be a one-off check
and we never had any intent to use the script again, the script never
needed to be added to the repo in the first place and hasn't been used
since 2021.