[configs] do not hardcode touchscreen devnode

On N9/N950 input devices do not guarantee to get assigned the same
devnode on every reboot. Once touchscreen might be /dev/input/event3,
the other time it can be /dev/input/event1.

So rely on autodetection (in prev commit it was false assumption that
autodetect doesn't work reliably)

conf/60-n9-n950-ui.conf

@@ -4,5 +4,5 @@ QT_QPA_EGLFS_DEPTH=32
 QT_QPA_EGLFS_HIDECURSOR=1
 M_FORCE_LOCAL_THEME=1
 LIPSTICK_OPTIONS=
-QT_QPA_EVDEV_TOUCHSCREEN_PARAMETERS="/dev/input/event3"
+QT_QPA_EVDEV_TOUCHSCREEN_PARAMETERS=
 PRE_USER_SESSION_DISPLAY_OPTIONS=-displayX 187 -displayY -187 -displayRotation -90