Automatically deleting lines within generated polygon using QGIS?

Question

How do I automatically delete lines within a generated polygon?

what I have: is a polygon with lines within. The lines I want to be deleted automatically -- like a auto clean up.

This is what I did:

1. opened a postcode shapefile from https://www.suche-postleitzahl.org
2. Selected features (Berlin postcodes)
3. Merged selected features
4. Struggling to find a way on how to clean up the polygon in the layer.

Example