Bug 1044103 - app splash screen transparency is not opaque enough (r=etienne)

Bug 1044103 - app splash screen transparency is not opaque enough (r=etienne)
yes, only the homescreen uses the fade-overlay, and the homescreen sets `opacity:1` so we can remove this.

can you also remove the homescreen special rule [1] now that it's not needed anymore? Thanks!

[1] https://github.com/etiennesegonzac/gaia/blob/91003c59ab1ddde6ebab4dad6f035ee05881a53b/apps/system/style/window.css#L211-213