exports.c: drop unused dummy function

exports.c: drop unused dummy function

The !CONFIG_PHY_AQUANTIA defines were already superfluous since
_exports.h does have a CONFIG_PHY_AQUANTIA, so the entries never
existed. In fact, it couldn't have worked, because the defines would
affect both occurences of the mdio_get_current_dev identifier in the

	EXPORT_FUNC(mdio_get_current_dev, struct mii_dev *,
		    mdio_get_current_dev, void)

so the C code would end up containing four copies of

  gd->jt->dummy = dummy

but struct jt_funcs would not and does not have any 'dummy' member.

Now that nothing in _exports.h refers to dummy(), remove the empty
function.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>