netdev: drop CONFIG_PPC_MERGE from Kconfig

Now that arch/ppc is dead CONFIG_PPC_MERGE is always defined for all
powerpc platforms so we don't need to depend on it.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>