Remove unnecessary dependencies to SharpDX and Content.Pipeline from the core library (#881)

* remove unused SharpDX using

introduces in #840

* remove Content.Pipeline reference from the core library

introduced in #692

source/MonoGame.Extended/Math/OrientedRectangle.cs

@@ -2,7 +2,6 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using Microsoft.Xna.Framework;
-using SharpDX.Direct3D;
 
 namespace MonoGame.Extended
 {

source/MonoGame.Extended/MonoGame.Extended.csproj

@@ -6,9 +6,6 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="MonoGame.Framework.Content.Pipeline"
-                      Version="3.8.1.303"
-                      PrivateAssets="All" />
   </ItemGroup>
 
 </Project>