[SPARC]: Remove legacy stuff from cpu_idle().

Currently sparc and sparc64's UP cpu_idle() checks current pid. This
is old time legacy. Now it's paranoia.

Signed-off-by: Coywolf Qi Hunt <coywolf@lovecn.org>
Acked-by: William Irwin <wli@holomorphy.com>
Signed-off-by: David S. Miller <davem@davemloft.net>