Remove unused compat variable SCREEN_DEV

Currently we set USE_SCREEN to SCREEN_DEV if it's set.  There is a
comment to remove it once it's eracticated from CI.

AFAICT this pre-condition has been met.

Change-Id: I1423c8b9c18d1b3e34dbfe1c03be735c646a12b4

Author: tbreeds

stackrc

@@ -651,9 +651,6 @@ S3_SERVICE_PORT=${S3_SERVICE_PORT:-3333}
 PRIVATE_NETWORK_NAME=${PRIVATE_NETWORK_NAME:-"private"}
 PUBLIC_NETWORK_NAME=${PUBLIC_NETWORK_NAME:-"public"}
 
-# Compatibility until it's eradicated from CI
-USE_SCREEN=${SCREEN_DEV:-$USE_SCREEN}
-
 # Set default screen name
 SCREEN_NAME=${SCREEN_NAME:-stack}