Remove draw line 2D helper method.

Callers can provide 3D points easily, and this avoids the need to allocate and slow down enumeration via the points.Select(p => new float3(p, 0)) wrapper.

OpenRA.Mods.Common/Widgets/LineGraphWidget.cs

@@ -131,7 +131,7 @@ namespace OpenRA.Mods.Common.Widgets
 						{
 							lastX = x;
 							lastPoint = point;
-							return origin + new float2(x * xStep, -point * scale);
+							return origin + new float3(x * xStep, -point * scale, 0);
 						}), 1, color);
 
 					if (lastPoint != 0f)