Remove dead DotNet40 reference

Remove dead reference to DotNet40 nuspec

Author: reisenberger

src/Polly.sln

@@ -6,7 +6,6 @@ MinimumVisualStudioVersion = 10.0.40219.1
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{79FE9DBE-1155-4F78-A41F-FE95DEFA19DD}"
 	ProjectSection(SolutionItems) = preProject
 		GlobalAssemblyInfo.cs = GlobalAssemblyInfo.cs
-		Polly.Net40Async.nuspec = Polly.Net40Async.nuspec
 		Polly.nuspec = Polly.nuspec
 	EndProjectSection
 EndProject