So I did some tests. First I had a look in .sch file :
F 2 is kicad reserved field
F 9 is the one I want to remove
In symbol field , I have selected all the footprint from the second field and deleted them.
But they are also automatically deleted in the other field.
I watched in the .sch file and I saw that it was F 2 line was deleted not F 9.
I restored my copy from the original project and tried to delete the other field first.
But I got the same result.
Done a new restore.
In symbol field , if I modifiy one of the footprint field the other one is automatically updated with the new value
In symbol propertie , if i delete the content of the second footprint, the first one (kicad reserved) stay the same .
I deleted the second field ,the first stay alive.
In the .sch file F 2 is still present but F 9 is empty but present (normal?)
So my conclusions are :
If I want to remove the second footprint in symbol field I must modify the file that manages that window. (but it is really not necessary because I only have to uncheck the field to not see it)
if I want to remove the real second footprint field, I must delete every F9 line of each part.
But I have almost 800 parts… If you have any idea to do it in a fast way…