Remove temporary workaround for reactor-netty client

Now that https://github.com/reactor/reactor-netty/issues/138 is solved,
this commit removes the temporary workaround introduced in SPR-15784
but leaves the general mechanism for detecting empty responses.

spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpConnector.java

@@ -38,12 +38,6 @@ import org.springframework.http.HttpMethod;
  */
 public class ReactorClientHttpConnector implements ClientHttpConnector {
 
-	private static final Mono<ClientHttpResponse> NO_CLIENT_RESPONSE_ERROR = Mono.error(
-			new IllegalStateException("HttpClient completed without a response. " +
-					"As a temporary workaround try to disable connection pool. " +
-					"See https://github.com/reactor/reactor-netty/issues/138."));
-
-
 	private final HttpClient httpClient;
 
 
@@ -78,8 +72,7 @@ public class ReactorClientHttpConnector implements ClientHttpConnector {
 				.request(adaptHttpMethod(method),
 						uri.toString(),
 						request -> requestCallback.apply(adaptRequest(method, uri, request)))
-				.map(this::adaptResponse)
-				.switchIfEmpty(NO_CLIENT_RESPONSE_ERROR);
+				.map(this::adaptResponse);
 	}
 
 	private io.netty.handler.codec.http.HttpMethod adaptHttpMethod(HttpMethod method) {