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/test/java/sample/webflux/SampleWebFluxApplicationTests.java

@@ -18,6 +18,7 @@ package sample.webflux;
 
 import org.junit.Test;
 import org.junit.runner.RunWith;
+import reactor.core.publisher.Mono;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
@@ -44,4 +45,14 @@ public class SampleWebFluxApplicationTests {
 				.expectBody(String.class).isEqualTo("Hello World");
 	}
 
+	@Test
+	public void testEcho() throws Exception {
+		this.webClient.post().uri("/echo")
+				.contentType(MediaType.TEXT_PLAIN)
+				.accept(MediaType.TEXT_PLAIN)
+				.body(Mono.just("Hello WebFlux!"), String.class)
+				.exchange()
+				.expectBody(String.class).isEqualTo("Hello WebFlux!");
+	}
+
 }