[HildonLiveSearch] Unset the filter visible func. during dispose
[HildonLiveSearch] Unset the filter visible func. during dispose

This prevents a crasher when the model changes after the live search is
disposed.

android-808/libhildon,Cordia/libhildon,android-808/libhildon,Cordia/libhildon,android-808/libhildon,Cordia/libhildon