Remove the events-correlation-engine plugin (#16885)

We [reached agreement][1] to move this plugin to a separate repository. The
implementation was never completed here, and this code was never backported to
any release, so it is safe to remove.

[1]: #7771 (comment)

Signed-off-by: Andrew Ross <[email protected]>

Author: andrross

gradle/missing-javadoc.gradle

@@ -170,7 +170,6 @@ configure([
   project(":libs:opensearch-common"),
   project(":libs:opensearch-core"),
   project(":libs:opensearch-compress"),
-  project(":plugins:events-correlation-engine"),
   project(":server")
 ]) {
   project.tasks.withType(MissingJavadocTask) {