docker-compose: remove unused ENV variable (#456)

docker-compose.yml

@@ -59,6 +59,3 @@ services:
       # This directory must have cert files if you want to enable SSL
       - ./volumes/web/cert:/cert:ro
       - /etc/localtime:/etc/localtime:ro
-    # Uncomment for SSL
-    # environment:
-    #  - MATTERMOST_ENABLE_SSL=true