Remove code related to anti-adblock notification

Background

We are currently working on removing the anti-adblock notification (see https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui/issues/43). That change will be made available as part of #96 (closed). However, we can already go ahead and stop importing that code and remove any other code that's specific to or only used by that notification.

What to change

• Stop using adblockplusui/lib/antiadblockInit.js.
• Stop importing translations from adblockplusui/locale/*/antiadblockInit.json.
• Remove code that preinstalls the "Adblock Warning Removal List" filter list.
• Deprecate the "subscriptions_antiadblockurl" preference.
• Remove code that's specific to "question"-type notifications.
• Remove code specific to "urlFilters" notification targeting option.

Hints for testers

With this change:

• The "Adblock Warning Removal List" filter list should no longer be automatically added.
• For users who already have that filter list, it should be left installed.
• If that filter list is present, it's still considered a default subscription like the others and so shouldn't prevent the default subscriptions from being automatically re-added.
• Notifications can no longer be of type "question", nor target specific URLs with the "urlFilters" option.
• Notifications should otherwise behave as before.