[libcalamaresui] Provide default implementations of next() and back()

- These methods are used for multi-page view-steps, which are rare.
   For all the others, just drop the empty implementation and defer
   to the base class.

src/modules/users/UsersViewStep.h

@@ -43,9 +43,6 @@ public:
 
     QWidget* widget() override;
 
-    void next() override;
-    void back() override;
-
     bool isNextEnabled() const override;
     bool isBackEnabled() const override;