Remove ScriptChannel

Since 1999be3484154d22f41afba465fb56366e6cf61b, the `ScriptChannel` hasn't been used anymore to get access to a script's content. However, it was still accessible to userscripts: If a resource was defined with a name ending in `.user.js`, `GM_getResourceText("example.user.js")` would work as expected (i.e. it would return the resource content), whereas accessing the URI returned by `GM_getResourceURL("example.user.js")`  would return the parent script's content.

An example script showing this unexpected behavior can be found in this gist: https://gist.github.com/Ventero/8640014

Thus, the special behavior of `greasemonkey-script:foo.user.js` URIs should be removed.