Fix crash caused by attempting to run a QgsMapLayerAction

... from the idenitify results dialog after the action has been removed. This commit fixes a potential crash which can caused by:
- enabling an atlas for a vector layer
- running the identify tool on that layer, so that the results dialog is shown
- while the dialog is shown, disable the atlas for that layer. The QgsMapLayerAction is removed, but the action isn't removed from the display identify results dialog
- Clicking on the invalid action in the identify results dialog crashes QGIS

This commit makes sure the QgsMapLayerAction is removed from the identify results dialog when it is destroyed.