Now, I'm a bit iffy about this since the earlier examples don't use that exact quote but instead are just instances of folks meeting a sudden left-field demise. After all, it's the quote that's the important bit, here.
I also found it cute that starfangx and I had come up with a similar line back in 1997. Ok, so in the start of Season Two of Transformers: Beast Wars, they had to add some new characters (due to pressure from Hasbro) so they had to ditch a few of the Season One characters to make room for them. There was a big earthquake sort of thing, there were some lava fissures on the floor of the antagonists' base, and two of the folks (Terrorsaur and Scorponok) just sort of fall off a cliff, land in the lava, melt down into slag, and are seen nevermore. Yes, these are characters who had (relatively speaking, since this is a Transformers cartoon) serious character growth and interaction in the previous season...but their demise is just "...and they fell in some lava and died." And to add insult to
After that, she and I took to citing "...they fell in some lava and died." whenever a character was just suddenly and mysteriously written out of a book/show/film/campaign/et cetera.