fix shellcheck failures of cluster/gce/gci/shutdown.sh

cluster/gce/gci/shutdown.sh

@@ -16,7 +16,7 @@
 
 # A script that let's gci preemptible nodes gracefully terminate in the event of a VM shutdown.
 preemptible=$(curl "http://metadata.google.internal/computeMetadata/v1/instance/scheduling/preemptible" -H "Metadata-Flavor: Google")
-if [ ${preemptible} == "TRUE" ]; then
+if [ "${preemptible}" == "TRUE" ]; then
     echo "Shutting down! Sleeping for a minute to let the node gracefully terminate"
     # https://cloud.google.com/compute/docs/instances/stopping-or-deleting-an-instance#delete_timeout
     sleep 30

hack/.shellcheck_failures

@@ -11,7 +11,6 @@
 ./cluster/gce/gci/flexvolume_node_setup.sh
 ./cluster/gce/gci/health-monitor.sh
 ./cluster/gce/gci/master-helper.sh
-./cluster/gce/gci/shutdown.sh
 ./cluster/gce/list-resources.sh
 ./cluster/gce/upgrade-aliases.sh
 ./cluster/gce/upgrade.sh