configure_system: Make GetAccountUsername() an internal function

We can just make the function accept an arbitrary ProfileManager
reference and operate on that instead of tying the function to the class
itself. This allows us to keep the function internal to the cpp file and
removes the need to forward declare the UUID struct.

src/yuzu/configuration/configure_system.h

@@ -9,17 +9,16 @@
 #include <QList>
 #include <QWidget>
 
-namespace Service::Account {
-class ProfileManager;
-struct UUID;
-} // namespace Service::Account
-
 class QGraphicsScene;
 class QStandardItem;
 class QStandardItemModel;
 class QTreeView;
 class QVBoxLayout;
 
+namespace Service::Account {
+class ProfileManager;
+}
+
 namespace Ui {
 class ConfigureSystem;
 }
@@ -36,7 +35,6 @@ public:
 
 private:
     void ReadSystemSettings();
-    std::string GetAccountUsername(Service::Account::UUID uuid) const;
 
     void UpdateBirthdayComboBox(int birthmonth_index);
     void RefreshConsoleID();