assets: Remove no longer used OSTC logo from graphic assets

This placeholder image has now been replaced with the project logo.

Signed-off-by: Stefan Schmidt <stefan.schmidt@huawei.com>

assets/ostc-logo-small-loading.png.license

-# SPDX-FileCopyrightText: Huawei Inc.
-#
-# SPDX-License-Identifier: Apache-2.0

assets/ostc-logo-small.png.license

-# SPDX-FileCopyrightText: Huawei Inc.
-#
-# SPDX-License-Identifier: Apache-2.0