We should remove Reader Mode on websites that don't need it

[kylehickinson]

Description

List of websites that clearly don't need Reader Mode showing up in the omnibox:

1. twitter.com
2. reddit.com
3. youtube.com
   This list is not exhaustive, we can add to it.

This is also a webcompat issue because we regularly get iOS webcompat reports that are sent while user is in Reader mode (we know this because we brave/brave-ios#8667). It's possible that the user accidentally hits the button (very easy to do) and gets stuck because they think the page is broken.

Note that ultimately we should move to Speedreader: brave/brave-ios#8690