Remove FlexRay standard reference

Author: Benedikt Menne

docs/5____additional_content.adoc

@@ -23,5 +23,3 @@ Contributions to this layered standard to the FMI standard are covered by the ht
 
 - [[[PW13]]] Preston-Werner, T. (2013): **Semantic Versioning 2.0.0**.  https://semver.org/spec/v2.0.0.html
 - [[[RFC2119]]] RFC 2119: **Key words for use in RFCs to Indicate Requirement Levels**. https://tools.ietf.org/html/rfc2119
-- [[[ISO-17458-2]]] ISO 17458-2:2013(en):
-**Road vehicles - FlexRay communications system - Part 2: Data link layer specification**. https://www.iso.org/standard/59806.html