Disable setting implicit value of a job's cpus_per_task value

This removes logic added three years ago that would automatically set a job's cpus_per_task value in order to reset a job's mem_per_cpu value and scale the cpus_per_task by the same value. Equivalent logic did not exist in the step allocation logic. Just return an error instead. This change will be made in Slurm version 2.6, but this batch is made for version 2.5. The original patch introducing the problem is in commit: cc00cc70bug 352