cmd: remove gen-manifests in favor of the images version

This is a result of the outcome of
osbuild#4124

Achilleas suggested that instead of having a copy of `gen-manifests`
in osbuild-composer we could use the one from images. AFAICT nothing
in the code references `gen-manifests` except the `test/README` so
it seems safe to remove and just run the one from images.

Note that osbuild/images#693 needs to
land before:
```
go run github.com/osbuild/images/cmd/gen-manifests@latest
```
works.