Remove the regions/en_US.json bootstrap file. This was necessary when
update_product_details was performing model validation, but since we
upgraded DMPD in 4438f0714b, it no longer
does, so this file is no longer needed.
You will still have to run update_product_details before doing anything
that _does_ require model validation, like runserver. This is probably
for the best.
Model validation, which happens before a management command can run,
unfortunately, fails with a totally empty product_details directory.
Bootstrap it by including the en_US.json region file.
Maintain the directory but prevent product_details data from being
checked into the repository. The build process already downloads updated
product details data.