Remove MonoTouch1.0 (classic Xamarin.iOS) from nuspec.

This configuration is officially deprecated and removed from nupkg in commit 3f3d480, so should be removed from nuspec too.

Author: yfakariya

MsgPack.nuspec

@@ -21,7 +21,6 @@ This package provides MessagePack serialization/deserialization APIs. This pacak
       <group targetFramework=".NETFramework3.5" />
       <group targetFramework="WindowsPhone8.0" />
       <group targetFramework="MonoAndroid1.0" />
-      <group targetFramework="MonoTouch1.0" />
       <group targetFramework="Xamarin.iOS1.0" />
       <group targetFramework=".NETStandard1.1">
         <dependency id="System.Collections" version="4.0.11" />