Doc: cleanup semaphore example

This patch removes unused signal and variable.

Change-Id: Ia4eaf083493d3d37e3ff22e0380d5a5ee69f91cf
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>

examples/corelib/threads/semaphores/semaphores.cpp

@@ -94,12 +94,6 @@ public:
         }
         fprintf(stderr, "\n");
     }
-
-signals:
-    void stringConsumed(const QString &text);
-
-protected:
-    bool finish;
 };
 //! [4]