Stop using remote localized files in CFR

Set browser.newtabpage.activity-stream.asrouter.useRemoteL10n to false to avoid:

08:14:37.308 NetworkError when attempting to fetch resource. ASRouter.jsm:110:8
    reportError resource://activity-stream/lib/ASRouter.jsm:110
    _remoteSettingsLoader resource://activity-stream/lib/ASRouter.jsm:336
    AsyncFunctionThrow self-hosted:696

Ref.: #40700 (comment 2760323) Rel.: https://bugzilla.mozilla.org/show_bug.cgi?id=1585434