Remove SCREEN_LOGDIR from devstack install setting

Screen support has been completely removed from devstack.
So we should remove this setting.

Change-Id: I65da9c0ead5b437338af55c6b95bf88a8295b08f
diff --git a/cinder/tests/tempest/README.rst b/cinder/tests/tempest/README.rst
index 71248f4..079321d 100644
--- a/cinder/tests/tempest/README.rst
+++ b/cinder/tests/tempest/README.rst
@@ -30,7 +30,6 @@
     MYSQL_PASSWORD=$ADMIN_PASSWORD
     RABBIT_PASSWORD=$ADMIN_PASSWORD
     SERVICE_PASSWORD=$ADMIN_PASSWORD
-    SCREEN_LOGDIR=/opt/stack/screen-logs
     LOGFILE=$DEST/logs/stack.sh.log
     LOGDAYS=2
     SYSLOG=False