Hygiene: Cleanup enwp.less prepare to destroy it

* .navbox is display:none in hacks.less yet there are loads of rules
in enwp.less that style it. Thus removing.
* other things are hidden in hacks.less so moving other display:none
rules from enwp.less there
* add FIXME to destroy enwp.less - originally it was designed to be
for enwiki specific styles but now we have Mobile.css and hacks.less
this has become confusing and redundant

Change-Id: I3eb546c869ff5db25f1ee0e8843149103302668e