'Gravemind' is an entity created from one of the later stages of Flood infection, usually spawning from large infestations. This particular Gravemind, the one prominently featured in Halos 2 and 3 as the de facto 'leader' of the parasite, is probably dead, or if it isn't, then the stage of infection has regressed far enough that it cannot maintain such a consciousness anymore. Installation 04-II's premature firing not only destroyed it, but also damaged the Ark, and, coupled with High Charity's destruction, led to the literal incineration of much Flood tissue. Coupled with the resulting Halo blast that would've wiped out any Covenant loyalists somewhere on the Ark, there would be literally nothing for the Flood to rebuild itself with. Whatever's left on the Ark is probably just a few infection forms, certainly nothing like the grand Cthulu-spawn that was present in the climax of Halo 3.