Menu: Actually hide PlatformNav if !hasKeys

Returning {} just leads to a present but blank entry

(Also, nobody notice that the capcheck was wrong inside platnav anyway ;p).

frontend/ui/elements/platform_navigation.lua

@@ -1,15 +1,9 @@
 local Device = require("device")
-
--- Everything in there requires physicla page turn keys ;).
-if Device:hasKeys() then
-    -- No menu entry at all if we don't have any
-    return {}
-end
-
 local Event = require("ui/event")
 local UIManager = require("ui/uimanager")
 local _ = require("gettext")
 
+-- This whole menu is hidden behind a hasKeys device cap.
 local PlatformNav = {
     text = _("Page turn behavior"), -- Mainly so as to differentiate w/ "Page Turns" when in readermenu...
     sub_item_table = {