Remove hardcoded mpspawn owner lint check

Use RequiresSpecificOwner to enforce the owner
that owns the world instead.

Require 'Neutral' in the official mods accordingly.

OpenRA.Mods.Common/Lint/CheckPlayers.cs

@@ -91,11 +91,6 @@ namespace OpenRA.Mods.Common.Lint
 					var ownerName = ownerInit.PlayerName;
 					if (!playerNames.Contains(ownerName))
 						emitError("Actor {0} is owned by unknown player {1}.".F(kv.Key, ownerName));
-					else if (kv.Value.Value == "mpspawn" && !players[ownerName].OwnsWorld)
-					{
-						emitError("Actor {0} needs to be owned by the player that owns the world. ".F(kv.Key) +
-							"Use the `Spawn` and `LockSpawn` player properties to force players onto a particular spawn instead.");
-					}
 
 					RequiresSpecificOwnersInfo info;
 					if (actorsWithRequiredOwner.TryGetValue(kv.Value.Value, out info))