Ok, you're talking about Firefox specifically; Camino's Page Info option is basic, and Safari doesn't even have one.
Regardless, the answer is the same--that's information about an external document, not something you have any control whatsoever over (though even if you did, I'm not clear on why you'd want to remove it). Asking how to clear it is sort of like renting a video and asking how to throw away the box that the rental place keeps on its shelf--you have no control over it, and its existence doesn't affect you one way or the other.
If it bothers you, you could always switch to Safari, which won't show you that data. Odd, though, since I've been developing websites since the dotcom era, and have only looked at that maybe once, and even then only by accident.