Remove remaining security slider code

Recently, we saw that some code related to security slider is still present: tpo/web/manual#40 (comment 2776978).

We should remove it and check that everything keeps working after that modification, especially security settings.

The whole JS file (chrome/content/preferences-mobile.js) seems not needed anymore as well as chrome/content/preferences.xhtml (that is still referenced by chrome/content/torbutton.js).