oniro-linux-demo-dashboard.conf: Remove deprecated distro configuration

This was long ago replaced by oniro-linux-blueprint-dashboard.conf and
kept for backwards compatible. We are good now to cleanup.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

meta-oniro-blueprints/conf/distro/oniro-linux-demo-dashboard.conf

-# SPDX-FileCopyrightText: Huawei Inc.
-#
-# SPDX-License-Identifier: Apache-2.0
-
-# Deprecated distro configuration in favour of
-# oniro-linux-blueprint-dashboard
-
-require conf/distro/oniro-linux-blueprint-dashboard.conf
-
-# We keep the old distro name to sanity check this deprecation.
-DISTRO = "oniro-linux-demo-dashboard"