cmd-buildextend-metal: remove --artifact from cosa meta call

From what I can tell this is only used in the case where we
are delaying meta.json merging until later. For example, when
parallel building and we want meta.json snippets (i.e. meta.aws.json)
to be what gets merged at the end.

Let's remove it from here to simplify.

Author: dustymabe

src/cmd-buildextend-metal

@@ -315,8 +315,7 @@ json.dump(j, sys.stdout, indent=4)
     fi
 
     # and now the crucial bits
-    cosa meta --workdir "${workdir}" --build "${build}" \
-        --artifact "${platform}" --artifact-json "$(readlink -f meta.json.new)"
+    cosa meta --workdir "${workdir}" --build "${build}" --artifact-json "$(readlink -f meta.json.new)"
     /usr/lib/coreos-assembler/finalize-artifact "${imgpath}" "${builddir}/${imgname}"
 
     # Quiet for the rest of this so the last thing we see is a success message