Remove input_balloon_time setting

This setting doesn't work on most platforms
including some notification daemons on linux.

Where it does work I think it is bad behavior
to default overriding their defaults anyway.

It's most valuable use was likely win32
tray balloons which no longer exist.

src/fe-gtk/notifications/notification-dummy.c

@@ -17,7 +17,7 @@
  */
 
 void
-notification_backend_show (const char *title, const char *text, int timeout)
+notification_backend_show (const char *title, const char *text)
 {
 }