Stop injecting custom b2g content stylesheet thanks to bug 874984. Fixes #495

Following platform b2g desktop bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=874984
is going to fix multiple things for us:
 * it will no longer apply to xul content, so that we no longer need to avoid applying the existing content.css file shipped in b2g desktop,
 * it fixes the scrollbar look'n feel in order to have exactly the same appearance that we have on device,
 * it also fixes scrollbar repaint issues reported in #495.

We should wait before landing this patch to either uplift this bug or cherry pick it on our build.