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I'm afraid that can't be done

(Well, not very easily anyway

)
If you're simply looking to reduce the number of nodes in the file, you could try Path menu > Simplify,
once. If you click it more than once, it will likely distort the paths. But that won't result in the simple paths you're looking for.
If you're just wanting simple paths, things get more complicated (ironically

). One option would involve a lot of node editing, which many people detest (although I rather enjoy it). When I say "a lot" I guess it really depends on the complexity of the raster image in the first place. Perhaps it might be done fairly quickly, if it's a very simple image. Node editing (look in manual under Node tool) involves breaking up those complex and usually multiple closed paths into the simpler components. I can give you detailed instructions, if you choose this option.
Another option depends on the image you're tracing. If it's like a line drawing, you could forget about Trace bitmap, and trace it "manually", i.e. using the Pen tool (or Pencil), and on a new layer above the raster image, draw fresh new lines, etc. Just like using tracing paper back in the day (or maybe like using carbon paper too)

You could reduce the opacity of the layer with the raster image, if it helps. This creates those simple paths from the start.
Let us know if these ideas will work
