Adapt WebFlux.fn auto-config to SPR-15536

Since SPR-15536, WebFlux.fn is now configured with `@EnableWebFlux`,
along the annotated controllers. Both `RouterFunction` and Controller
instances can now live within the same application and they share
the same web infrastructure.

This commit removes the custom auto-configuration for `RouterFunction`
and relies on `@EnableWebFlux` for that.

Closes gh-9165

spring-boot-samples/spring-boot-sample-webflux/src/main/java/sample/webflux/EchoHandler.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2012-2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package sample.webflux;
+
+import reactor.core.publisher.Mono;
+
+import org.springframework.stereotype.Component;
+import org.springframework.web.reactive.function.server.ServerRequest;
+import org.springframework.web.reactive.function.server.ServerResponse;
+
+@Component
+public class EchoHandler {
+
+	public Mono<ServerResponse> echo(ServerRequest request) {
+		return ServerResponse.ok().body(request.bodyToMono(String.class), String.class);
+	}
+
+}

spring-boot-samples/spring-boot-sample-webflux/src/main/java/sample/webflux/SampleWebFluxApplication.java

@@ -18,6 +18,12 @@ package sample.webflux;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.context.annotation.Bean;
+import org.springframework.web.reactive.function.server.RouterFunction;
+import org.springframework.web.reactive.function.server.ServerResponse;
+
+import static org.springframework.web.reactive.function.server.RequestPredicates.POST;
+import static org.springframework.web.reactive.function.server.RouterFunctions.route;
 
 @SpringBootApplication
 public class SampleWebFluxApplication {
@@ -26,4 +32,9 @@ public class SampleWebFluxApplication {
 		SpringApplication.run(SampleWebFluxApplication.class);
 	}
 
+	@Bean
+	public RouterFunction<ServerResponse> monoRouterFunction(EchoHandler echoHandler) {
+		return route(POST("/echo"), echoHandler::echo);
+	}
+
 }