Stop toggling the FAKE flag for stringified GVs

Stop toggling the FAKE flag for stringified GVs

Back in the old days, when GVs had magic attached to them, get-magic
used to stringify the glob and assign the string to the PV field.  It
was triggered in mg.c like this:

    gv_efullname3(sv,((GV*)sv), "*");

Because sv was both the glob being read *and* the string being
assigned to, the stringification would call set-magic on the glob and
flatten the glob into a string.  Or something like that.

That is not necessary any more.