Remove assert from Http3Connection.SendAsync (#74348) (#74484)

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3Connection.cs

@@ -240,7 +240,7 @@ public async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, lon
             catch (QuicException ex) when (ex.QuicError == QuicError.OperationAborted)
             {
                 // This will happen if we aborted _connection somewhere and we have pending OpenOutboundStreamAsync call.
-                Debug.Assert(_abortException is not null);
+                // note that _abortException may be null if we closed the connection in response to a GOAWAY frame
                 throw new HttpRequestException(SR.net_http_client_execution_error, _abortException, RequestRetryType.RetryOnConnectionFailure);
             }
             finally