Bug 46441: Move legacy css for menu into mobilemenu.css

I was reviewing Ic6fd7585d25a051895e1eea26acb2f4934c97120 to suggest
this change but it got merged before I could do so

the jsEnabled method is flawed. It also applies to phones with js enabled
Older phones can not access the menu without going via Special:MobileMenu
page - so putting the fallback css here makes much more sense.

In process fix..
Bug 46441

Change-Id: I4365f914f95a8b4d1f2d9ab2e37614e835cc2fc5
Merge "Bug 46441: Move legacy css for menu into mobilemenu.css"