remove references to SingletonAttribute (#962)

Author: omsmith

src/D2L.CodeStyle.Analyzers/Attributes.cs

@@ -5,7 +5,6 @@ namespace D2L.CodeStyle.Analyzers {
 
 	internal static class Attributes {
 
-		internal static readonly RoslynAttribute Singleton = new RoslynAttribute( "D2L.LP.Extensibility.Activation.Domain.SingletonAttribute" );
 		internal static readonly RoslynAttribute DIFramework = new RoslynAttribute( "D2L.LP.Extensibility.Activation.Domain.DIFrameworkAttribute" );
 		internal static readonly RoslynAttribute Dependency = new RoslynAttribute( "D2L.LP.Extensibility.Activation.Domain.DependencyAttribute" );
 

src/D2L.CodeStyle.Analyzers/Extensions/Microsoft.CodeAnalysis.cs

@@ -6,19 +6,6 @@
 namespace D2L.CodeStyle.Analyzers.Extensions {
 	internal static partial class RoslynExtensions {
 
-		public static bool IsTypeMarkedSingleton( this ITypeSymbol symbol ) {
-			if( Attributes.Singleton.IsDefined( symbol ) ) {
-				return true;
-			}
-			if( symbol.Interfaces.Any( IsTypeMarkedSingleton ) ) {
-				return true;
-			}
-			if( symbol.BaseType != null && IsTypeMarkedSingleton( symbol.BaseType ) ) {
-				return true;
-			}
-			return false;
-		}
-
 		private static readonly SymbolDisplayFormat FullTypeDisplayFormat = new SymbolDisplayFormat(
 			typeQualificationStyle: SymbolDisplayTypeQualificationStyle.NameAndContainingTypesAndNamespaces,
 			miscellaneousOptions: SymbolDisplayMiscellaneousOptions.ExpandNullable