Qt: traffic table: remove a tap listener only once

Qt: traffic table: remove a tap listener only once

Class TrafficTableTreeWidget is used by the conversation and enpoint
dialogs, both of which are subclasses of WiresharkDialog.

Those dialogs use WiresharkDialog::registerTapListener() to register tap
listeners. When the dialog is closed, those listeners are removed by
WiresharkDialog::removeTapListeners().

TrafficTableTreeWidget's destructor tries to remove its tap listener a
2nd time after WiresharkDialog did its cleanup. This causes warnings

Warn remove_tap_listener(): no listener found with that tap data

Don't call remove_tap_listener() from TrafficTableTreeWidget's
destructor. The destructor is now empty and can be removed completely.

Change-Id: I3143fa1c5116203f4a0be791bd4c5f08135aefb0
Reviewed-on: https://code.wireshark.org/review/31259
Petri-Dish: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Petri Dish Buildbot
Reviewed-by: Jaap Keuter <jaap.keuter@xs4all.nl>
Reviewed-by: Michael Mann <mmann78@netscape.net>