Remove MapReactorSessionRepository default constructor

This commit removes the default `MapReactorSessionRepository` so that the users are required to explicitly supply the `Map` used to store the sessions.

samples/javaconfig/webflux/src/main/java/sample/HelloWebfluxSessionConfig.java

@@ -16,6 +16,8 @@
 
 package sample;
 
+import java.util.concurrent.ConcurrentHashMap;
+
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.session.EnableSpringWebSession;
@@ -29,7 +31,7 @@ public class HelloWebfluxSessionConfig {
 
 	@Bean
 	public MapReactorSessionRepository reactorSessionRepository() {
-		return new MapReactorSessionRepository();
+		return new MapReactorSessionRepository(new ConcurrentHashMap<>());
 	}
 }
 // end::class[]