remove the useless env var DISABLE_BOOT_IMAGE_UPDATE in 4.19 to 4.20 upgrade job (#66441)

Author: gpei

ci-operator/config/openshift/openshift-tests-private/openshift-openshift-tests-private-release-4.20__amd64-nightly-4.20-upgrade-from-stable-4.19.yaml

@@ -267,7 +267,6 @@ tests:
     env:
       AWS_INSTALL_USE_MINIMAL_PERMISSIONS: "yes"
       BASE_DOMAIN: qe.devcluster.openshift.com
-      DISABLE_BOOT_IMAGE_UPDATE: "true"
     test:
     - chain: openshift-upgrade-qe-test
     workflow: cucushift-installer-rehearse-aws-ipi-workers-marketplace
@@ -316,7 +315,6 @@ tests:
     cluster_profile: aws-usgov-qe
     env:
       BASE_DOMAIN: qe.devcluster.openshift.com
-      DISABLE_BOOT_IMAGE_UPDATE: "true"
     test:
     - chain: openshift-upgrade-qe-test
     workflow: cucushift-installer-rehearse-aws-usgov-ipi-private-workers-marketplace