Nissan Motor Corp.'s formidable 300ZX is the latest sports car to bite the dust in the U.S., yet another victim of high prices, brutal insurance rates and changing buyer demographics. The No. 2 Japanese automaker says suspension of exports to the U.S. -- known as the Fairlady Z in Japan -- is the result of poor sales and changes required by new and stricter U.S. emission standards. But the entire sports car segment -- a powerhouse in the mid-'80s -- is literally evaporating. The highly regarded Toyota MR2 was killed off earlier this year after sales dwindled from 32,314 in 1985 to only 259 in the first half of '95. Among the other great cars which might also soon be killed off: the Mazda RX7, Acura NSX and even the Ford Probe.