powerpc/configs: NET_CADENCE became NET_VENDOR_CADENCE

The NET_CADENCE symbol was renamed to NET_VENDOR_CADENCE, so we don't
need to disable the former, see commit 0df5f81c ("net: ethernet:
Add missing VENDOR to Cadence and Packet Engines symbols").

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20200121043000.16212-2-mpe@ellerman.id.au