Split off last bot modules

And dissolve AI namespace.
There would have been so little left in Common.AI,
that keeping it made no sense anymore.

OpenRA.Mods.Common/Traits/BotModules/HarvesterBotModule.cs

@@ -13,7 +13,6 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using OpenRA.Mods.Common.Activities;
-using OpenRA.Mods.Common.AI;
 using OpenRA.Mods.Common.Pathfinder;
 using OpenRA.Traits;