Remove ScriptChannel.

Using the resource URL of a resource with a name that ends with
.user.js would lead to surprising results: GM_getResourceText
would return the actual resource content, but using the URL returned
by GM_getResourceURL would result in the parent script content being
returned.

Refs 1999be3484154d22f41afba465fb56366e6cf61b.
Remove FF3 specific code path.