Change Session None to use PHP session configuration instead of trying to force file mode

Currently, using the set_ini function to force file usage for sessions will only work if PHP is already configured to store sessions in files.  If it is configured for a different session storage, such as memcached, the session_path variable points to the session server, not a file path, and thus setting file mode breaks when it tries to save to a file using the session path variable.   

My hypothesis is that at one point in time that set_ini was used in JSession in order to change the PHP session handling process and that this set_ini was used to put PHP back into file mode[blindly assuming it was in file mode to begin with].

With the current PHP session handling function registration method, that hypothetical process was replaced and this set_ini call was left lying around by mistake.  Since most PHP configurations just use file session handling, this ini call "works" for 95% of sites, but causes problems on the 5% using alternate methods.

I don't see how removing it can cause any problems for the 95% and will fix things for the 5% - as such this fixes Joomla! Session handling for everyone.