Remove explicitcancel option

[antonok-edm]

Related brave-core work - brave/brave-core#6632

The Brave browser has previously returned an empty response with HTTP 200 for blocked requests, and used the $explicitcancel option to actually fail the requests. While this is a cool unique capability offered by integrating the adblocker inside a browser rather than as an extension, it actually forces more maintenance overhead because most community adblock lists are not built with this behavior in mind. Some scripts have different behavior when a particular network request is blocked rather than successful with an empty body.

We're updating Brave to always cancel the requests, just like other adblockers - which means the $explicitcancel option has no meaning and should be removed.

[antonok-edm]

Removed in 7811e54.