The hotkey for automated Pollution ('damage') cleanup is Shift-D in Conquests; it also works for lava. This hotkey (along with several others), got changed from Vanilla/ PtW, in part I think because of the new Terrain-types (e.g. the 'Automate: Clear Jungle' hotkey also got changed from Shift-J to Shift-W for 'Clear Wetlands' since the command now also applies to Marsh).
If you go into the 'Preferences' menu, and switch on 'Show Advanced Unit Action Buttons', you should then be able to see what the actual hotkey is in-game, by rolling the cursor over each one of the 'automate' buttons.
There was also a Conquests cheat-sheet PDF prepared by Julian Egelstaff (plus others), and downloadable from here on CFC -- but I can't find it at the moment (possibly it got lost in the big server-move last year). It is(/was) very good for the most part, and has(/had) a fairly comprehensive list of the commands, but unfortunately was not fully updated from PtW: ironically, the 'Clear Pollution' hotkey was one of the few things that was not amended