Do not attempt to lookup task program in slurmd.

Reverts most of commit 84023f27.

Searching the PATH in slurmd can fail due to root_squash'd NFS
filesystems, leading to the "wrong" program being launched.

If you'd like the performance benefit from avoiding this lookup
during each separate task launch, set SLURM_TEST_EXEC=1 instead
which will perform the lookup once within srun, which then ensures
the lookup happens under the users own environment and not that
of the slurmd.

Bug 2992.