EF6.0 “The relationship could not be changed because one or more of the foreign-key properties is non-nullable”

You need to delete the ItemTypeItem.
It is not possible to just remove it from the Items list as it cannot exist by itself, because it has a non-nullable foreign key referencing ItemType (ItemTypeID).

To delete the ItemTypeItem add

context.Entry(itemTypeItem).State = EntityState.Deleted;