Don't change the default session_handler if it is allready set Ref: #5127

#### Steps to reproduce the issue
We are using memcache as default session handler and this set up is in php.ini.

#### Expected result
Joomla to use default session handler is not set in configuration.

#### Actual result
PHP Warning:  session_start(): open(tcp://127.0.0.1:11211/sess_r5c5940ecaru55pr69bssctmu2, O_RDWR) failed: No such file or directory


#### System information (as much as possible)
php.ini: session.save_handler = memcache


#### Additional comments
Change libraries/joomla/session/storage/none.php to use default php session handler.

See: #5127 by @Olvikolvi