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.UnitTesting/Shared/Maths/Matrix3_Test.cs

@@ -3,7 +3,6 @@ using System.Collections.Generic;
 using System.Numerics;
 using NUnit.Framework;
 using Robust.Shared.Maths;
-using Vector3 = Robust.Shared.Maths.Vector3;
 
 namespace Robust.UnitTesting.Shared.Maths
 {

Robust.UnitTesting/Shared/Prototypes/PrototypeManager_Test.cs

@@ -10,8 +10,6 @@ using Robust.Shared.Prototypes;
 using Robust.Shared.Serialization.Manager;
 using Robust.Shared.Serialization.Manager.Attributes;
 using Robust.Shared.Serialization.TypeSerializers.Implementations.Custom.Prototype.Array;
-using Vector3 = Robust.Shared.Maths.Vector3;
-using Vector4 = Robust.Shared.Maths.Vector4;
 
 namespace Robust.UnitTesting.Shared.Prototypes
 {

Robust.UnitTesting/Shared/Serialization/NetSerializer_Test.cs

@@ -111,7 +111,7 @@ namespace Robust.UnitTesting.Shared.Serialization
             {
                 CustomTypeSerializers = new ITypeSerializer[]
                 {
-                    new Vector2Serializer(),
+                    new NetMathSerializer(),
                 }
             });
             var stream = new MemoryStream();

Robust.UnitTesting/Shared/Toolshed/ToolshedParserTest.Core.cs

@@ -44,8 +44,8 @@ public sealed partial class ToolshedParserTest
                 // maff
                 AssertParseable<Vector2>();
                 AssertParseable<Vector2i>();
-                AssertParseable<Robust.Shared.Maths.Vector3>();
-                AssertParseable<Robust.Shared.Maths.Vector4>();
+                AssertParseable<Vector3>();
+                AssertParseable<Vector4>();
                 AssertParseable<Box2>();
                 AssertParseable<Box2i>();
                 AssertParseable<Angle>();