I believe in 3E, although its possible that the trend may have started in Planescape during the 2E years, but I'm pretty sure the complete dropping of "prime" was in third.
/off topic
Shemeska...I like yur new avatar...when did you change it?
Oh! I thought when all the sub-primes went bust, there was no need to keep the "prime" designation. You know, recession and all that.total wild hairbrained guess:
when Hasbro bought D&D.
they didn't want to confuse Optimus and the other Primes with anything D&D.
I'm not even sure if 4e uses it the term at all, since I've seen it simply refer to it as "the world" (which I find bizarre since it would seem to imply one a single planet Prime, which is gimpy).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.