I have this complex vector illustration and the lines overlap without a solid fill. Instead, I wanted to "flatten" or merge the lines so it's just one vector. Instead of it being filled, I want it to be transparent. If I remove the fill it shows the lines of the other vectors.
In principle your image is like part 1 in my next image:
It has numerous filled paths with black stroke and also some unfilled black strokes. The filled areas partially cover each other and the strokes. You want to get a single black shape 2 and remove what's in 3.
One way to get it is to select all and convert the strokes to filled areas by applying Object > Path > Create outlines.
Then Select all and apply Pathfinder panel's Merge. That makes all visible black areas one and removes the invisible black areas.
Ungroup and move apart or delete the fills (=3)
If there's some isolated black shapes you can select all and make a group or you can apply Object > Compound Path > Make or Pathfinder Panel's Unite to combine the pieces to a single item. Group is the only easily reversible way to make a combination. Object > Compound Path > Release will generate a mess.