Reverting pid file patch

https://github.com/sysown/proxysql/issues/2494 

For now users should manually update the systemd unit file in case of a non-default ProxySQL deployment

systemd/system/proxysql.service

@@ -9,7 +9,7 @@ RuntimeDirectory=proxysql
 #ExecStartPre=/usr/bin/mkdir -p /var/run/proxysql /var/run/proxysql
 #ExecStartPre=/usr/bin/chown -R proxysql: /var/run/proxysql/
 ExecStart=/usr/bin/proxysql -c /etc/proxysql.cnf
-#PIDFile=/var/lib/proxysql/proxysql.pid
+PIDFile=/var/lib/proxysql/proxysql.pid
 #StandardError=null  # all output is in stderr
 SyslogIdentifier=proxysql
 Restart=no