Remove unnecessary constraints

Remove unnecessary constraints.

This merge proposal was created automatically by the Janitor bot. For more information, including instructions on how to disable these merge proposals, see https://janitor.debian.net/scrub-obsolete.

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The bot will automatically update the merge proposal to resolve merge conflicts or close the merge proposal when all changes are applied through other means (e.g. cherry-picks). Updates may take several hours to propagate.

Build and test logs for this branch can be found at https://janitor.debian.net/scrub-obsolete/pkg/cssparser/6cbf1865-cccb-407c-92b7-c8a4b6156211.

These changes have no impact on the binary debdiff.

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