The mitkDisplayInteractor has been replaced by the mitkDisplayActionEventBroadcast here: 251e40159685
The mitkDisplayInteractor is not required or used anymore, but some references still exist. This is because classes need to check for the InteractionEventObserver and see if this is of type mitkDisplayInteractor (mitkDisplayActionEventBroadcast) (e.g. see here: https://phabricator.mitk.org/source/mitk/browse/develop/Modules/Segmentation/Interactions/mitkTool.cpp$150-169).
Attention: QtFreeRender seems to create a mitkDisplayInteractor so we need to check if we can replace it here.

However, InteractionEventObserver should never be of type mitkDisplayInteractor, since this class is not used anymore.
Using this class would look something like this (example for mitkDisplayActionEventBroadcast): https://phabricator.mitk.org/source/mitk/browse/develop/Modules/QtWidgets/src/QmitkAbstractMultiWidget.cpp$49-53

The DisplayInteractor.cpp has been removed completely.