Fix packaging which broke because of asciidocs comments (elastic#3825)

The version.asciidoc is used during packaging to read out the version number. Recently a comment was introduce in the file which broke the packaging. In before_build.sh the content of the file is copied over and converted to yaml, see https://github.com/elastic/beats/blob/master/dev-tools/packer/xgo-scripts/before_build.sh#L58. As these lines to not seem to be required, I removed them. Alternatively the sed script could be adapted which would be more complex.

libbeat/docs/version.asciidoc

@@ -1,8 +1,4 @@
 :stack-version: 5.3.0
 :doc-branch: 5.3
 :go-version: 1.7.4
-
-//////////
-release-state can be: released | prerelease | unreleased
-//////////
 :release-state: released