The test manifests don't have any use in this repository, since the
osbuild-image-tests command has been removed. Likewise, generating
testing manifests is no longer needed in this repository, therefore
remove the gen-manifests command.
Manifests are being generated in the osbuild/images repository and the
necessary tooling to generate them lives there.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>