make: krb5 image already built by reana-dev

* Addresses reanahub/reana-auth-krb5#1.

Makefile

@@ -164,12 +164,6 @@ release-build:
 		-u "${GITHUB_USER}" -t auto \
 		--output-component-versions $$BUILT_IMAGES_FILE_PATH \
 		$(addprefix --exclude-components , ${EXCLUDE_COMPONENTS}) && \
-	echo "Building reanahub/krb5 image" && \
-	cd ../reana-job-controller && \
-	REANA_KRB5_IMAGE_NAME="reanahub/krb5:latest" && \
-	docker build --file krb5/Dockerfile --tag $$REANA_KRB5_IMAGE_NAME . && \
-	echo $$REANA_KRB5_IMAGE_NAME >> $$BUILT_IMAGES_FILE_PATH && \
-	cd ../reana && \
 	echo "The following images have been built:" && \
 	./scripts/update_images.sh $$BUILT_IMAGES_FILE_PATH && \
 	rm $$BUILT_IMAGES_FILE_PATH