delete superfluous environment variables

dockerfiles/images/redis/Dockerfile

@@ -1,8 +1,6 @@
 FROM   	    stackbrew/ubuntu:saucy
 MAINTAINER  Meaglith Ma <genedna@gmail.com> (@genedna), Lance Ju <juzhenatpku@gmail.com> (@crystaldust)
 
-ENV DEBIAN_FRONTEND noninteractive
-
 ENV         DEBIAN_FRONTEND noninteractive
 
 RUN         apt-get update && apt-get install -y redis-server