Hi JHH,
Fellow user here. Grain of salt. All that.
For any data item type, you can’t delete those greyed-out lines because they’re an option available to you for creating a new data item (of whatever type panel you’re on). This is in addition to the + Create dropdown, which is limited to the data types relevant to the panel you’re on, or the plus mark to the left, which offers every data item type available. So many options!
The greyed-out lines can be at different levels, as in your screenshot, if the item can be nested within itself (e.g., story within a story, character group within a character group, etc.) or within a parent (more below) so that you have the option to input new child items at any level within that structure.
Items with parent types will also show the parent types in the listing, e.g., character groups are in the characters panel along with the characters themselves. You can create a new character within a group (greyed-out lines within groups) or outside of any group (very bottom greyed-out line).
Hope this helps. :0)