[workspace] Use qDeleteAll

Rather than a foreach with questionable indentation

Differential Revision: https://phabricator.kde.org/D28334

workspace.cpp

@@ -501,8 +501,7 @@ Workspace::~Workspace()
     delete startup;
     delete Placement::self();
     delete client_keys_dialog;
-    foreach (SessionInfo * s, session)
-    delete s;
+    qDeleteAll(session);
 
     // TODO: ungrabXServer();