Remove exising signatures and do not sign .sig files in _sign_netboot_artifacts

_mk_pgp_signature and _cms_sign_artifact both delete the signature files so that it does not cause issues in re-runs. _sign_netboot_artifacts does not. Any existing sig files should be removed for consistency.

_sign_netboot_artifacts also globs all vmlinuz-"* files which will match any vmlinuz-"*".sig" files too. Since extglob is enabled, the glob pattern can be changed to "${_dir}${arch}/vmlinuz-"!(*.sig).