[Task]: Remove dead code/strings in statistics

[diox]

Description

The statistics panel we offer to developers haven't seen major changes in years, and it contains a bunch of code and strings that are completely unused.

This is noteworthy because it contains over 160 translated strings in JavaScript code, almost 10% of the total number of translated strings for addons-server! The vast, vast majority of those are useless (everything about application sales, refunds etc - that was meant for Firefox OS Marketplace back in the day - but also a lot from the time AMO had global statistics available for admins through that same UI, and a bunch about download sources that got removed 5 years ago in #7940) and are causing translators to do extra work for nothing.

We should do a quick pass to remove a bunch of that dead code, particularly translated strings.

Acceptance Criteria

• Dead code is removed from statistics

┆Issue is synchronized with this Jira Task