url: remove invalid file protocol check

'file' should have been 'file:' but since the WHATWG URL spec
suggests using an opaque origin (which is what was already being
used for file URLs), we'll just keep using that, making this merely
a cleanup.

PR-URL: nodejs#11691
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/url.js

@@ -1043,7 +1043,6 @@ function originFor(url, base) {
     case 'https:':
     case 'ws:':
     case 'wss:':
-    case 'file':
       origin = new TupleOrigin(protocol.slice(0, -1),
                                url[context].host,
                                url[context].port,