Revert "package list URL for Reactor javadoc in Dokka"

This reverts commit dc2535516c.
The Reactor project reverted its Javadoc changes.

gradle/docs.gradle

@@ -73,7 +73,6 @@ dokka {
 	}
 	externalDocumentationLink {
 		url = new URL("http://projectreactor.io/docs/core/release/api/")
-		packageListUrl = new URL("http://projectreactor.io/docs/core/release/api/element-list")
 	}
 	externalDocumentationLink {
 		url = new URL("http://www.reactive-streams.org/reactive-streams-1.0.1-javadoc/")