disable System Add-on updates (#595)

remember the new Coverage Telemetry shit? with a **hidden** opt-out pref? guess what, they are already collecting for 3 months ...

https://bugzilla.mozilla.org/show_bug.cgi?id=1487578 - **3 months ago**: "I see data coming in that looks reasonable"

guess what else ...

"It has also replaced the previous version that was there (from bug 1480194)" and oh, surprise surprise, 1480194 is ACCESS DENIED!

they're not just using private tickets to hide security critical information from potential hackers and blackhats, no they also use it to hide shady AF things. Things that they fully know are shady as fuck and that they absolutely know a lot of people would not like. There's simply no other reason why they'd do that

but wait, that's not all. If you think an opt-out pref that 99% of people wouldn't know about even if it showed up in about:config BUT ALSO HAPPENS TO BE HIDDEN is kind of questionable, well ... the system addon that they use for this shit apparently looked or still looks for `toolkit.telemetry.coverage.opt-out` [1] instead of `toolkit.coverage.opt-out` as their documentation [2] claims

[1] https://github.com/mozilla/one-off-system-add-ons/pull/131/files#diff-6e0cbf76986d04383ccb32a29ef27a7aR25
[2] https://hg.mozilla.org/mozilla-central/file/tip/toolkit/components/telemetry/docs/data/coverage-ping.rst#l32

It's time to opt out of all that shit for good. Disable system addon updates and kill it at the root

> In FF61 and lower, you will not get any System Add-on updates except when you update Firefox

on its own that's not true. You will get SA updates unless you disable app update checks + auto install. Let's just remove that as well.

user.js

@@ -385,10 +385,9 @@ user_pref("network.allow-experiments", false);
 user_pref("app.normandy.enabled", false);
 user_pref("app.normandy.api_url", "");
 user_pref("app.shield.optoutstudies.enabled", false);
-/* 0505: disable System Add-on updates
- * [NOTE] In FF61 and lower, you will not get any System Add-on updates except when you update Firefox ***/
-   // user_pref("extensions.systemAddon.update.enabled", false); // [FF62+]
-   // user_pref("extensions.systemAddon.update.url", "");
+/* 0505: disable System Add-on updates ***/
+user_pref("extensions.systemAddon.update.enabled", false); // [FF62+]
+user_pref("extensions.systemAddon.update.url", ""); // [FF44+]
 /* 0506: disable PingCentre telemetry (used in several System Add-ons) [FF57+]
  * Currently blocked by 'datareporting.healthreport.uploadEnabled' (see 0333) ***/
 user_pref("browser.ping-centre.telemetry", false);