ProjectExplorer: Remove unused Project::displayName() signal

Only ever emitted by the cmake project and never consumed

Change-Id: I689120a2ac2c77896eed72da9bd0be194ac1af86
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/cmakeproject.cpp

@@ -185,7 +185,6 @@ void CMakeProject::updateProjectData(CMakeBuildConfiguration *bc)
 
     updateQmlJSCodeModel();
 
-    emit displayNameChanged();
     emit fileListChanged();
 
     emit bc->emitBuildTypeChanged();