invalid sessions logging PHP errors into webserver log file

There is a funny line in \includes\main\WebUI.php which says that it logs something for development, every time a session is invalid. Since I am checking logfiles on regular base those unneccessary log entries only make it hard to check the log file. A debug log would here be the sufficient thing to do, like it's being done everywhere else. I will commit the changes and add a pull request

The errors are quite ugly looging like this:

[Mon Apr 18 15:04:07.706613 2016] [:error] [pid 26244:tid 140171735451392] [client 192.168.10.20:53901] FastCGI: server "/usr/lib/cgi-bin/php5-fcgi" stderr: PHP message: #0 /var/lib/shared/websites/crm/www/includes/main/WebUI.php(174): Vtiger_WebUI->checkLogin(Object(Vtiger_Request))
[Mon Apr 18 15:04:07.706648 2016] [:error] [pid 26244:tid 140171735451392] [client 192.168.10.20:53901] FastCGI: server "/usr/lib/cgi-bin/php5-fcgi" stderr: #1 /var/lib/shared/websites/crm/www/index.php(18): Vtiger_WebUI->process(Object(Vtiger_Request))
[Mon Apr 18 15:04:07.706653 2016] [:error] [pid 26244:tid 140171735451392] [client 192.168.10.20:53901] FastCGI: server "/usr/lib/cgi-bin/php5-fcgi" stderr: #2 {main}