Step-by-step guide
This can be done by setting the key "deletePV" as shown:
% grib_set -s deletePV=1 in.grib out.grib
Which is in fact equivalent to setting the number of coordinate values to 0 and clearing the "pv" array:
% grib_set -s PVPresent=0,NV=0 in.grib out.grib