Remove local $_ from Exporter

Remove local $_ from Exporter

This was added in commit 732bb7c2d4f.

The changes in that commit did not necessitate the addition of
local $_.  So the localisation is wasting CPU cycles.  Worse, it
causes bugs in 5.12 and earlier.  (local $_ is always wrong if you
don’t control what is in $_ already, because it could be a read-only
tied variable.)  Actually, it causes bugs in 5.14-15 still, because
it seems that the changes to ‘local $_’ still weren’t sufficient (it
still calls FETCH, but not STORE).  That itself needs fixing, but that
should not obviate the need for this change, as Exporter has been liv-
ing a double life.