ci: remove --exclusive from Phoenix CPU jobs

--exclusive caused 'Requested node configuration is not available' on
cpu-small,cpu-medium,cpu-large partitions. The --mem-per-cpu=8G
reservation already prevents memory contention from co-scheduled jobs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/scripts/submit-slurm-job.sh

@@ -87,8 +87,7 @@ if [ "$device" = "cpu" ]; then
             sbatch_device_opts="\
 #SBATCH -p cpu-small,cpu-medium,cpu-large
 #SBATCH --ntasks-per-node=12
-#SBATCH --mem-per-cpu=8G
-#SBATCH --exclusive"
+#SBATCH --mem-per-cpu=8G"
             ;;
         frontier|frontier_amd)
             sbatch_device_opts="\