Do not alter an unused inlet based on appearance alone. Manufacturers have specific arrangements for optional connections, and opening a sealed connection can be irreversible or create an unintended leak path.
What the observation tells you
A moulded stub is not necessarily a blocked drain that needs clearing. Similarly, a cap or plug found nearby should not be removed to improve grinding or ventilation. Installation drawings explain its purpose, but they do not make modification an ordinary owner-maintenance task.
A useful next step
If you suspect the wrong connection was used, photograph the accessible arrangement and ask the responsible installer or engineer to assess it. Keep the appliance unmodified meanwhile.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.