[DOCS] Remove comments around ES 7.4 release highlights include (elastic#570)

Author: jrodewig

docs/en/install-upgrade/highlights.asciidoc

@@ -43,13 +43,11 @@ include::{beats-repo-dir}/highlights-7.4.0.asciidoc[tag=notable-highlights]
 This list summarizes the most important enhancements in {es} {version}.
 For the complete list, go to {ref}/release-highlights.html[{es} release highlights].
 
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 :leveloffset: +1
 
 include::{es-repo-dir}/reference/release-notes/highlights-7.4.0.asciidoc[tag=notable-highlights]
 
 :leveloffset: -1
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 [[kibana-higlights]]
 === {kib} highlights