Returning your first example is pretty simple, your second format I don't believe is possible straight out from maya, but this would be achievable with a bash script or many other regex scripts, for your first required output:
PyMel: in the script editor, just select your obj(s) and run this code:
from pymel.core import *
for object in selected():
polyMapDel(object.f)
File > Export > OBJExport:
Result:
f 9//25 10//26 12//27 11//28
f 11//29 12//30 14//31 13//32
f 13//33 14//34 16//35 15//36
Or to do this without the use of a script, simply select the model and change your menu set to 'Polygons', in the 'Edit UVs' menu, you will see toward the bottom, 'Delete UVs'.
This above will also return the same result, Hope this helps