gv_fetchpvn_flags: Simplify some warnings code

This line originally checked isLEXWARN_on and had two different code
paths depending on that -- one of which called ckWARN(), which already
checks for isLEXWARN_off, so rather than doing duplicate work, we
just call ckWARN().
gv.c: Explain the purpose of the <none>:: stash a bit.

It's primarily a historical oddity. Its main purpose is to store
variables after an exception from 'use strict "vars"':

    eval q{ use strict; $foo = 1 };
    warn keys %{"<none>::"};