chore: remove reference to RHIDP-451 as we have closed it as obsolete (#845)

Author: nickboldt

README.adoc

@@ -24,5 +24,3 @@ Submit a pull request against this repo.
 
 ## Plugins Reference Guide 
 For Plugin configuration guide, see upstream content in https://github.com/janus-idp/backstage-plugins/tree/main/plugins
-
-TODO: transform backstage-plugins markdown content into adoc, hosted in this repo. See https://issues.redhat.com/browse/RHIDP-451 for details.