LanguageClient: improve callhierarchy direction items

Do not use the icon of the root function for the artificial "Incoming"
and "Outgoing" items in the call hierarchy. In the case of deprecated
functions this can be a warning icon.

Change-Id: Ib75a66296210d4da24208e98efb527b9af37b793
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/languageclient/callhierarchy.cpp

@@ -141,6 +141,8 @@ public:
     {
         if (role == Qt::DisplayRole)
             return m_direction == Incoming ? Tr::tr("Incoming") : Tr::tr("Outgoing");
+        if (role == Qt::DecorationRole)
+            return {};
         return CallHierarchyItem::data(column, role);
     }
 };