Don't run gtk-update-icon-cache when uninstalling the Qt package.  But do run
it when installing the gnome package.

Change-Id: I0bbd9c0edc3fb6fe0e672aa3dbea5ba23d847d87
Reviewed-on: https://code.wireshark.org/review/5598
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>