Remove unsupported attribute for local.conf in docs

Since the attribute 'SCREEN_LOGDIR' has been removed from patch[1]
So, remove it in local.conf described in doc file.

[1] https://review.openstack.org/#/c/499186


Change-Id: Ib6eca058da5b1ead64d907d71daf89d57e575608

doc/source/contributor/development-environment-devstack.rst

@@ -65,7 +65,6 @@ configuration:
     RABBIT_PASSWORD=password
     LOGFILE=$DEST/logs/stack.sh.log
     LOG_COLOR=False
-    SCREEN_LOGDIR=$DEST/logs
     LOGFILE=$DEST/logs/stack.sh.log
     LOGDAYS=1