There is a new way to do it in UE 5.5.
You have to enable modeling plugin for this to work.
First of all u click on the mesh you need to delete extra slots from go to modeling mode
Then you go to edit materials and under materials section there is a trash bin button that will delete all the material slots there are. If you wish to delete individual material slot click the arrow to the right of the material and a menu with delete prompt will be availiable.
