Makefile: remove Go 1.9.7

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

Makefile

@@ -65,16 +65,13 @@ endif
 
 # Example:
 #   GO_VERSION=1.8.7 make build-docker-test
-#   GO_VERSION=1.9.7 make build-docker-test
 #   make build-docker-test
 #
 #   gcloud docker -- login -u _json_key -p "$(cat /etc/gcp-key-etcd-development.json)" https://gcr.io
 #   GO_VERSION=1.8.7 make push-docker-test
-#   GO_VERSION=1.9.7 make push-docker-test
 #   make push-docker-test
 #
 #   gsutil -m acl ch -u allUsers:R -r gs://artifacts.etcd-development.appspot.com
-#   GO_VERSION=1.9.7 make pull-docker-test
 #   make pull-docker-test
 
 build-docker-test: