Convert use of _nc() to _nx()

[nacin]

_c() was deprecated for _x(). Core still uses _nc() in one spot, however. Patch moves that over to _nx().

Patch also formally deprecates _nc() and moves it to deprecated.php, next to its sibling _c().

I imagine other deprecated moves can be handled in #11388 for 3.0.

I really want to do this.

But not this close to release - really trying to avoid unnecessary string changes.

(In [12459]) Deprecate _nc() as _nx() is a much better was of specifying contexts. Fixes #11404 props nacin.