Clean up obsolete preferences in our 000-tor-browser.js

A cypherpunk noticed some preferences settings that could get cleaned up. Notably:

pref("browser.policies.testing.disallowEnterprise", true);
pref("extensions.legacy.exceptions",
"{972ce4c6-7e08-4474-a285-3208198ce6fd},torbutton@torproject.org");

But there might be more. E.g. we could go over the WebGL settings as we know that there is no pref way anymore for disabling extensions (see: #40117) while we still set pref("webgl.disable-extensions", true);