Removing some extension methods.

AsyncRx.NET/System.Reactive.Async/System/Reactive/Linq/Operators/Cast.cs

@@ -17,7 +17,7 @@ namespace System.Reactive.Linq
 
     partial class AsyncObserver
     {
-        public static IAsyncObserver<TSource> Cast<TSource, TResult>(this IAsyncObserver<TResult> observer)
+        public static IAsyncObserver<TSource> Cast<TSource, TResult>(IAsyncObserver<TResult> observer)
         {
             if (observer == null)
                 throw new ArgumentNullException(nameof(observer));

AsyncRx.NET/System.Reactive.Async/System/Reactive/Linq/Operators/Contains.cs

@@ -29,7 +29,7 @@ namespace System.Reactive.Linq
 
     partial class AsyncObserver
     {
-        public static IAsyncObserver<TSource> Contains<TSource>(this IAsyncObserver<bool> observer, TSource element)
+        public static IAsyncObserver<TSource> Contains<TSource>(IAsyncObserver<bool> observer, TSource element)
         {
             if (observer == null)
                 throw new ArgumentNullException(nameof(observer));
@@ -37,7 +37,7 @@ namespace System.Reactive.Linq
             return Contains(observer, element, EqualityComparer<TSource>.Default);
         }
 
-        public static IAsyncObserver<TSource> Contains<TSource>(this IAsyncObserver<bool> observer, TSource element, IEqualityComparer<TSource> comparer)
+        public static IAsyncObserver<TSource> Contains<TSource>(IAsyncObserver<bool> observer, TSource element, IEqualityComparer<TSource> comparer)
         {
             if (observer == null)
                 throw new ArgumentNullException(nameof(observer));

AsyncRx.NET/System.Reactive.Async/System/Reactive/Linq/Operators/DefaultIfEmpty.cs

@@ -25,7 +25,7 @@ namespace System.Reactive.Linq
 
     partial class AsyncObserver
     {
-        public static IAsyncObserver<TSource> DefaultIfEmpty<TSource>(this IAsyncObserver<TSource> observer)
+        public static IAsyncObserver<TSource> DefaultIfEmpty<TSource>(IAsyncObserver<TSource> observer)
         {
             if (observer == null)
                 throw new ArgumentNullException(nameof(observer));
@@ -33,7 +33,7 @@ namespace System.Reactive.Linq
             return DefaultIfEmpty(observer, default(TSource));
         }
 
-        public static IAsyncObserver<TSource> DefaultIfEmpty<TSource>(this IAsyncObserver<TSource> observer, TSource defaultValue)
+        public static IAsyncObserver<TSource> DefaultIfEmpty<TSource>(IAsyncObserver<TSource> observer, TSource defaultValue)
         {
             if (observer == null)
                 throw new ArgumentNullException(nameof(observer));

AsyncRx.NET/System.Reactive.Async/System/Reactive/Linq/Operators/OfType.cs

@@ -17,7 +17,7 @@ namespace System.Reactive.Linq
 
     partial class AsyncObserver
     {
-        public static IAsyncObserver<TSource> OfType<TSource, TResult>(this IAsyncObserver<TResult> observer)
+        public static IAsyncObserver<TSource> OfType<TSource, TResult>(IAsyncObserver<TResult> observer)
         {
             if (observer == null)
                 throw new ArgumentNullException(nameof(observer));