Remove extension based content script cosmetic filtering

[bbondy]

We should remove some of these content-script based (via brave-extension) rules since I believe they should be covered by cosmetic filters.

https://github.com/brave/brave-core/blob/master/components/brave_extension/extension/brave_extension/manifest.json#L56

The full assets are here:
https://github.com/brave/brave-core/tree/master/components/brave_extension/extension/brave_extension/assets

The extension way isn't good because:

• It adds manual things we can't dynamically update
• It does not work on Android
• It should be covered already by cosmetic filters

Test plan

Test to make sure with an older build vs this build, that the following sites block the same:

• https://www.washingtonpost.com/
• https://www.youtube.com/
• https://www.yahoo.com/
• https://coinmarketcap.com/
• https://www.glennbeck.com/

[rebron]

@bbondy can you assign a priority on this one? p4?

[antonok-edm]

@bbondy / @rebron Looks like this can be closed? brave/brave-core#5386 (comment)

[bbondy]

Yes but it was via brave/brave-core#5878
Dupe of #10319