Help isolating stave and exporting a midi file ?

I'm trying to isolate and export one staff.

I have tried to copy/paste the staff onto a new document.

I have tried to make a copy of the original document and delete all the voices except the one line in the staff for which I need the midi file --just the melody line.

Yet every time I import a midi file into the software program and play it (Hookpad), all the parts from the deleted staves also play although they are not represented visually in the program.

I hope that I'm clear. Files below.
Thank you anyone 😁