ProjectExplorer: Make BuildStepConfigWidget::displayName a value

Use BuildStep::displayName() as default.

This probably could be the only possibility, but currently there
are some discrepancies that are kept in this patch to make
the patch mechanical.

Change-Id: I2a1e5c2ff37ad95e25309eb16e07099e42191f60
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/ios/iosdsymbuildstep.cpp

@@ -267,11 +267,6 @@ IosDsymBuildStepConfigWidget::~IosDsymBuildStepConfigWidget()
     delete m_ui;
 }
 
-QString IosDsymBuildStepConfigWidget::displayName() const
-{
-    return m_buildStep->displayName();
-}
-
 void IosDsymBuildStepConfigWidget::updateDetails()
 {
     BuildConfiguration *bc = m_buildStep->buildConfiguration();