Clean up some custom awaiters (#48722)

Some appear to be dead code, others can be replaced by use of ConfigureAwaitOptions.  I did not replace ones that appeared to be used by signalr client code as well, as that would have required ifdef'ing.

Author: stephentoub

src/Servers/Kestrel/Core/src/Internal/Infrastructure/ThreadPoolAwaitable.cs

@@ -548,7 +548,7 @@ protected async Task InitializeConnectionAsync(RequestDelegate application, int
         InitializeConnectionWithoutPreface(application, addKestrelFeatures);
 
         // Lose xUnit's AsyncTestSyncContext so middleware always runs inline for better determinism.
-        await ThreadPoolAwaitable.Instance;
+        await Task.CompletedTask.ConfigureAwait(ConfigureAwaitOptions.ForceYielding);
 
         await SendPreambleAsync();
         await SendSettingsAsync();

src/Servers/Kestrel/test/InMemory.FunctionalTests/Http2/Http2TestBase.cs

@@ -238,7 +238,7 @@ private async Task ExecuteAsync(HttpContext context, ConnectionDelegate connecti
                     // Wait for the transport to run
                     // Ignore exceptions, it has been logged if there is one and the application has finished
                     // So there is no one to give the exception to
-                    await connection.TransportTask!.NoThrow();
+                    await ((Task)connection.TransportTask!).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
 
                     // If the status code is a 204 it means the connection is done
                     if (context.Response.StatusCode == StatusCodes.Status204NoContent)

src/SignalR/clients/csharp/Http.Connections.Client/src/Internal/TaskExtensions.cs

@@ -8,7 +8,6 @@
   <ItemGroup>
     <Compile Include="$(SignalRSharedSourceRoot)BinaryMessageFormatter.cs" Link="BinaryMessageFormatter.cs" />
     <Compile Include="$(SignalRSharedSourceRoot)BinaryMessageParser.cs" Link="BinaryMessageParser.cs" />
-    <Compile Include="$(SignalRSharedSourceRoot)ForceAsyncAwaiter.cs" Link="ForceAsyncAwaiter.cs" />
     <Compile Include="$(SignalRSharedSourceRoot)TextMessageFormatter.cs" Link="Internal\TextMessageFormatter.cs" />
     <Compile Include="$(SignalRSharedSourceRoot)TextMessageParser.cs" Link="Internal\TextMessageParser.cs" />
     <Compile Include="..\..\common\Http.Connections\src\ServerSentEventsMessageFormatter.cs" Link="ServerSentEventsMessageFormatter.cs" />

src/SignalR/common/Http.Connections/src/Internal/HttpConnectionDispatcher.cs

@@ -48,6 +48,7 @@ public override void Complete(Exception exception = null)
 
     public async Task<FlushResult> ForceAsyncResult()
     {
-        return await Task.FromResult<FlushResult>(default).ForceAsync();
+        await Task.CompletedTask.ConfigureAwait(ConfigureAwaitOptions.ForceYielding);
+        return default;
     }
 }