I think just removing the offending line will solve the issue.   It's my guess that the only reason Joomla has this line in it is because in the ancient past there was a line somewhere in the primary JSession class which called ini_set('session.save_handler', 'somevalue') in order to redirect the session saves to itself - so as a fallback SessionNone was created which would set it back to the 'default' value - where default was assumed to be file.  When we went to using session_set_save_handler, this one line of code was not removed due to an oversight.   

At least, I can think of no reason WHY we would want to set session.save_handler to files today.   If it was originally files, it remains files and the path is correct.  If it was something else, then the save_path is likely not pointing to a directory so setting it to files will break session handling.    

In either case, if SessionNone is a fallback for no other working session method, there is no need to override the default ini variable - either PHP is configured correctly and sessions will work, or it is not.  

I also suspect it's an oversight mainly because it only affects the 1% of people who are using a session handling extension for PECL such as APC or Memcache and have changed their configuration.  So it just never gets fixed because the number of people it affects is miniscule.