Fix clang error from de7eac9a.

Clang warns about a possible null dereference of job_part_ptr
if the !job_ptr->priority_array part of the conditional is taken.

Remove that part of the conditional, as it doesn't matter if that is
set or not here. The jobs eligibility on one vs. multiple partition is
not determined by that, but by the status of part_ptr_list and part_ptr.

Bug 5136.