Qt: Obey main toolbar icon preferences.

Remove the preferences for the filter toolbar and mark them GTK+-only.

Change-Id: Ie48b19aee29a1cfcea4c41ca6c08ddbba3102377
Reviewed-on: https://code.wireshark.org/review/5693
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>