Don't get rid of it. I actually haven't played Halo 3 yet, but in Halo 2, skulls were little rewards that could be found on higher difficulties in hard-to-find locations that could do some amusing things. Among those amusing things were to get rid of your HUD, make you invisible, jump super high, etc. In Halo 3, the function is probably the same. I don't know what the particular skull you found does, but keep it and search on Google for the name of the level you found it on followed by skull. (ie. "levelName skull")