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

[ShivanKaul]

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 wrongly report the internal URL). 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: #8690