Remove dead JavaScript code and strings from statistics

[diox]

Fixes: mozilla/addons#15874

Context

A lot of this code has been dead for years:

• We used to have webapps, some of them paid, through Firefox OS Marketplace. That is long gone, all related strings and code is obsolete
• We used to have global stats displayed using that code, also long gone
• We used to have download sources constants that we used and developers could use to track downloads with the src parameter, now it's using utm parameters that are completely different

Description

This removes the dead code and strings associated with those obsolete features. In total that's 114 strings that were translated but completely useless.

Testing

• Follow the (updated) Test Statistics Dashboard runbook to get stats working (including downloads)
• Browse through each item in the statistics dashboard and ensure everything loads correctly
• Repeat in a language with 100% strings translated like French and ensure everything is translated correctly

[eviljeff]

(unconnected, but we should do a better job of documenting how to enable localization - make compile_locales, etc - I had a head scratching moment when nothing was in French.)