I think you're wrong. That line existed on the first versions of LD and VHS (as in the original movie). It was the only way for Ben (Obi-Wan) to know how to shut down the force field. The line was put back in the SE versions and, I believe, in the Faces LD and VHS (1997).
No, actually Patrick was right...Though it was in the script, and probably recorded along with the rest of the movie, for whatever reason it got left out of the original theatrical prints. It was not in any released version of the film until (I think) the '81 re-release. It was also left out of the first couple vhs releases, and not included until (I think) the '84 tapes. Then it was overlooked again for the Definitive Collection Laserdiscs, and I think the next vhs set in that time period used the same transfer and skipped it again. But it's been included ever since.
So, I'm with Patrick. It certainly does add something to the logic of the movie, but it's been missing from so many different releases over the years that I'm fine with it missing from these sets as well. Maybe someday the ultimate bootleg version will take the trouble to seamlessly loop that in from some other source.