Remove some unused namespaces

Source/Core/Config/ResourceTextureSet.cs

Source/Core/Controls/ArgumentBox.cs

Source/Core/Controls/FieldsEditorControl.cs

Source/Core/Controls/MultiSelectTreeview.cs

Source/Core/Controls/ThingBrowserControl.cs

Source/Core/GZBuilder/Data/BoundingBox.cs

Source/Core/Geometry/Line3D.cs

Source/Core/Geometry/Tools.cs

Source/Core/Plugins/Plug.cs

Source/Core/Rendering/Renderer.cs

Source/Core/Rendering/Renderer2D.cs

Source/Core/Rendering/Renderer3D.cs

Source/Core/Rendering/ShaderManager.cs

Source/Core/Rendering/SurfaceManager.cs

Source/Core/Rendering/VisualVertexHandle.cs

Source/Core/Windows/MainForm.cs

Source/Core/Windows/TagStatisticsForm.cs

Source/Core/Windows/ThingStatisticsForm.cs

Source/Plugins/BuilderModes/ErrorChecks/ResultMissingFlat.cs

Source/Plugins/BuilderModes/ErrorChecks/ResultUnknownFlat.cs