net: thunderx: remove effective "default y" from Kconfig if ARCH_THUNDER=y

As well as for kernels built only for ThunderX ARCH_THUNDERX is also enabled
for kernels which support multiple platforms (such as distro kernels). Thus
"default ARCH_THUNDER" is inappropriate.

I believe default m is equally frowned upon, so remove the line completely
rather than "default m if ARCH_THUNDER".

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Cc: Sunil Goutham <sgoutham@cavium.com>
Cc: Robert Richter <rric@kernel.org>
Cc: Derek Chickles <derek.chickles@caviumnetworks.com>
Cc: Satanand Burla <satananda.burla@caviumnetworks.com>
Cc: Felix Manlunas <felix.manlunas@caviumnetworks.com>
Cc: Raghu Vatsavayi <raghu.vatsavayi@caviumnetworks.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>