Prevent sending of "remember-me" cookie deletion request when cookie not set

The "remember-me" cookie is deletion request is sent even when the cookie was never set - e.g. if the "Remember me" functionality is disabled in the Joomla configuration.

#### Steps to reproduce the issue
1. Log into the back-end of Joomla (www.example.com/administrator/).
2. Open browser tools for checking headers received with server response
3. Log out of the back-end
4. Check headers sent by browser and received with server response to GET www.example.com/administrator/index.php?option=com_login&task=logout&2048602984502834069720345=1 (the one with the "303 See other" response)

#### Expected result
4.. The session cookie sent on the request is requested to be deleted in the response.

#### Actual result
4.. Additionally to the session cookie, another cookie (the "remember-me" cookie) which had not been present in the request is also deleted in the response.

#### System information (as much as possible)
PHP 5.3.6 on Apache on Linux

#### Additional comments
This seems to be Joomla 2.5.x specific (and 1.5.x before that). On Joomla 3.x the "remember-me" functionality has been rewritten and the issue does not seem to occur (checked on the demo site http://joomla32.cloudaccess.net/administrator/index.php?autologin=1&passwd=demo&username=demo).

Also: added the HTTPS and HttpOnly cookie flags, as "Cookies must be deleted with the same parameters as they were set with" according to https://php.net/manual/en/function.setcookie.php. And that is how the cookie is set - see lines 739 - 740.