Pref: Remove unsued function

Fix also warning: no previous prototype for 'prefs_get_uint_preference' [-Wmissing-prototypes]

Change-Id: I4626372a9c131584c515a43e73aa019e2f696d29
Reviewed-on: https://code.wireshark.org/review/4960
Reviewed-by: Anders Broman <a.broman58@gmail.com>