[6.5] Removes security from Stack Overview (#631)

Author: lcawl

docs/en/stack/index.asciidoc

@@ -20,8 +20,6 @@ include::{asciidoc-dir}/../../shared/attributes.asciidoc[]
 
 include::introduction.asciidoc[]
 
-include::security/index.asciidoc[]
-
 include::ml/index.asciidoc[]
 
 include::{es-repo-dir}/ccr/index.asciidoc[]

docs/en/stack/limitations.asciidoc

@@ -13,12 +13,8 @@ For a list of known issues in the 6.0.0 alpha release, see
 https://www.elastic.co/blog/elastic-stack-6-0-0-alpha1-released[Elastic Stack 6.0.0-alpha1 Released].
 ////
 
-* <<security-limitations>>
-
 * <<ml-limitations>>
 
 --
 
-include::security/limitations.asciidoc[]
-
 include::ml/limitations.asciidoc[]