Remove extra copies of licenses and license headers

Remove extra copies of licenses and license headers

Remove the extra copy of the GPL license and license copyright headers
from CONTRIBUTING and the top-level COPYING.

Mention of the LICENSES/ directory and also mention the SPDX tag.

SPDX support is still in progress and COPYING files in subdirectories
still need to be updated.

Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>