workqueue: Remove unused work_on_cpu_safe

The last use of the work_on_cpu_safe() macro was removed recently by
commit 9cda46ba ("crypto: n2 - remove Niagara2 SPU driver")

Remove it, and the work_on_cpu_safe_key() function it calls.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Tejun Heo <tj@kernel.org>