sparc64: Kill __ARCH_WANT_UNLOCKED_CTXSW

As Peter Z. explained at:

https://lkml.org/lkml/2011/12/12/268

it's not needed at all and I even tested it back then.
This patch just got lost in the shuffle for some reason.

Reported-by: Kirill Tkhai <tkhai@yandex.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>