Remove Dead code --- BigInteger (#7511)

* Remove Dead code --- BigInteger

* Put back FSharp.Math namespace ... even though it's empty

* Improve empty namespace comment

Author: KevinRansom

src/fsharp/FSharp.Core/FSharp.Core.fsproj

@@ -122,12 +122,6 @@
     <Compile Include="reflect.fs">
       <Link>Reflection/reflect.fs</Link>
     </Compile>
-    <Compile Include="math\n.fsi">
-      <Link>Numerics/n.fsi</Link>
-    </Compile>
-    <Compile Include="math\n.fs">
-      <Link>Numerics/n.fs</Link>
-    </Compile>
     <Compile Include="math\z.fsi">
       <Link>Numerics/z.fsi</Link>
     </Compile>

src/fsharp/FSharp.Core/fslib-extra-pervasives.fsi

@@ -11,7 +11,6 @@ module ExtraTopLevelOperators =
     open Microsoft.FSharp.Control
     open Microsoft.FSharp.Collections
     open Microsoft.FSharp.Text
-    open Microsoft.FSharp.Math
 
 #if !FX_NO_SYSTEM_CONSOLE
     /// <summary>Print to <c>stdout</c> using the given format.</summary>