Remove redundant custom math types (#6078)

Vector3, Vector4, Matrix4, and Quaternion are now gone. Use System.Numerics instead.

This commit is just replacing usages, cleaning up using declarations, and moving over the (couple) helpers that are actually important.

Robust.Shared/Serialization/RobustSerializer.cs

@@ -89,8 +89,7 @@ namespace Robust.Shared.Serialization
                 CustomTypeSerializers = new[]
                 {
                     MappedStringSerializer.TypeSerializer,
-                    new Vector2Serializer(),
-                    new Matrix3x2Serializer(),
+                    new NetMathSerializer()
                 }
             };
             _serializer = new Serializer(types, settings);