In the interface directly if you goto Tools > Advanced Search > Find and Edit Max CPCs; under 3, specify changes, there's a radio button for removing keyword level CPCs.
In the editor, select all campaigns from the left hand menu. Goto the keyword tab and select all. Under the maximum CPC - just clear it. To make sure the changes were made, unselect all and select all the keywords again. The Maximum CPC should now be <default>.