Remove unreachable Firefox refresh button from download_firefox() Jinja macro

[escattone]

mozilla/sumo#1716

This PR is the deeper solution to mozilla/sumo#1716. The more surface level fix is #6137. If we ever want to spend the effort to make the Refresh Firefox button work as intended in the future, then this PR should be closed and #6137 reviewed/merged instead.

This PR arose from a deeper look from the symptom noted in mozilla/sumo#1716. Here's what I found:

• The Refresh Firefox button created by the download_firefox() Jinja macro is never shown, and can never be shown for multiple reasons. It has been this way for more than 6 years.
• First of all, the container of the Refresh Firefox button is never shown except when displayed in a browser that is not Firefox.
• When running in a non-Firefox browser, that container is shown, but the hidden Refresh Firefox button relies on the code in products.js to check if it should be shown, but that code, in turn, relies on a call to UITour, which only works when running in Firefox. So it's impossible for that specific Refresh Firefox button to ever be shown.
• Not only that, but the code in products.js relies on the latest Firefox version to be added as a data attribute on the Download Firefox button, but that data attribute has not been provided for some time.
• So this PR deletes products.js and the Refresh Firefox button portion of the download_firefox() Jinja macro.