create-spdx: delete virtual/kernel dependency to fix FreeRTOS build

When building FreeRTOS this dependency generates an error because
bitbake cannot find any provider for "virtual/kernel".
>From a dependency analysis the task is executed independently from
this so it can be safely removed.

This patch has been discussed in this ML thread:
https://lists.openembedded.org/g/openembedded-core/topic/90602531



Signed-off-by: Davide Gardenal <davide.gardenal@huawei.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>