sched: remove extern declaration of msm_sched_setaffinity()

msm_sched_setaffinity() function is not defined in the scheduler.
Remove the extern declaration from the header file.

Change-Id: I015e498e128c30f731f7f6e23cb432a87b709fc2
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>