ui: let DEs control notifications timeouts

There have been some issues (#673) informing that the notifications
timeout were not working on KDE.

On 843412d I wrote that the timeout unit is millisecond, as stated on
the docs here:
https://notify2.readthedocs.io/en/latest/#notify2.Notification.set_timeout

But after some trial and error:
 - set_timeout() units are in seconds, at least for KDE 5.26.3, Xfce
   4.18 and GNOME 43.
 - not specifying the timeout with set_timeout() lets the Desktop
   Environment handle the timeout for us, from their respective
   preferences window.

So at least now there're some DEs where the notifications are closed as
expected.

ui/opensnitch/notifications.py

@@ -93,14 +93,6 @@ def show(self, title, body, icon="dialog-information", urgency=URGENCY_NORMAL, c
         try:
             ntf = self.ntf2.Notification(title, body, icon)
 
-            # timeouts seems to be ignored (on Cinnamon at least)
-            timeout = self._cfg.getInt(Config.DEFAULT_TIMEOUT_KEY, 15)
-            # -1 and 0 are special values
-            if timeout > 0:
-                timeout = timeout * 1000
-            ntf.set_timeout(timeout * 1000)
-            ntf.timeout = timeout * 1000
-
             ntf.set_urgency(urgency)
             ntf.set_category(self.CATEGORY_NETWORK)
             # used to display our app icon and name.