Remove default IMAGE_TAG

It points to a non existent commit

Makefile

@@ -120,7 +120,7 @@ terraform-init: bin/terrafile set-azure-account
 		-backend-config=storage_account_name=${AZURE_RESOURCE_PREFIX}${SERVICE_SHORT}tfstate${CONFIG_SHORT}sa \
 		-backend-config=key=${CONFIG}.tfstate
 
-	$(if $(IMAGE_TAG), , $(eval export IMAGE_TAG=sha-194cbc9))
+	$(if $(IMAGE_TAG), , $(error The IMAGE_TAG variable must be provided))
 	$(eval export TF_VAR_paas_app_docker_image=ghcr.io/dfe-digital/get-into-teaching-api:$(IMAGE_TAG))
 	$(eval export TF_VAR_azure_resource_prefix=$(AZURE_RESOURCE_PREFIX))
 	$(eval export TF_VAR_config_short=$(CONFIG_SHORT))