softlockup: remove unused definition for spawn_softlockup_task

The definition of spawn_softlockup_task in sched.h became
unnecessary once it was converted to the early_initcall()
interface.

Signed-off-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>