Remove .NET 6 from nuspec

Author: slozier

Package/nuget/IronPython.nuspec

@@ -29,10 +29,6 @@ This package contains the IronPython interpreter engine.</description>
         <dependency id="Microsoft.Win32.Registry" version="4.7.0" />
         <dependency id="Mono.Unix" version="7.1.0-final.1.21458.1" />
       </group>
-      <group targetFramework="net6.0">
-        <dependency id="DynamicLanguageRuntime" version="1.3.5" />
-        <dependency id="Mono.Unix" version="7.1.0-final.1.21458.1" />
-      </group>
       <group targetFramework="net8.0">
         <dependency id="DynamicLanguageRuntime" version="1.3.5" />
         <dependency id="Mono.Unix" version="7.1.0-final.1.21458.1" />