ci: remove unreachable job_id guard after retry_sbatch

Under set -e, retry_sbatch returning 1 exits the script immediately.
The if [ -z "$job_id" ] block was never reachable.

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

Author: sbryngelson

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

@@ -197,11 +197,6 @@ EOT
 job_id=$(retry_sbatch "$_sbatch_script")
 unset _sbatch_script
 
-if [ -z "$job_id" ]; then
-    echo "ERROR: Failed to submit job."
-    exit 1
-fi
-
 echo "Submitted batch job $job_id"
 echo "$job_id" > "$id_file"
 echo "Job ID written to $id_file"