Revert "Make slurmd oom-unkillable by default, using oom.h OOM_SCORE_ADJ_MIN macro."

This mostly reverts commit 8325e9f8.

Bug 4478

We decided to not do this by default for the slurmd.  See bug 4478 for
more info/reasoning.

We also didn't want to rely on #include <linux/oom.h> since that might
not exist everywhere.