cleanup: removes uneeded annotation when not using network policy (#1884)

#### What type of PR is this?

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Add one of the following kinds:
/kind bug
/kind documentation
/kind feature
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/kind cleanup

#### What this PR does / why we need it:

Removes the network policy annotation which is not needed (we are not
generating network policy)

#### Which issue(s) this PR fixes:
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*Automatically closes linked issue when PR is merged.
Usage: `Fixes #<issue number>`, or `Fixes (paste link of issue)`.
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Fixes https://github.com/kubernetes/kompose/issues/1759

#### Special notes for your reviewer:

script/test/fixtures/single-file-output/output-k8s.yaml

@@ -28,7 +28,6 @@ spec:
   template:
     metadata:
       labels:
-        io.kompose.network/single-file-output-default: "true"
         io.kompose.service: front-end
     spec:
       containers: