Fix/api v2 shared versions with id parameter

https://www.openproject.org/work_packages/15077

First commit is just code cleanup and removing an accept_key_auth statement to a non existing action (show). Second commit removes an unnecessary visibility scoping (please see https://github.com/opf/openproject/commit/331fab456e33426e7e0d5e085a2144dde9a5342a for reasons).