Fix compiler warning about missing return statement in non-void function

Found by ICC 17 Beta on Windows.

This commit also removes the "default" cause from the switch, which will
make compilers let us know if we add another enum but don't handle it in
the switch.

Change-Id: Ib57b52598e2f452985e9fffd1458b5b39b238db7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/gui/kernel/qshortcutmap.cpp

@@ -353,9 +353,9 @@ bool QShortcutMap::tryShortcut(QKeyEvent *e)
         // shouldn't say that we handled the event.
         return identicalMatches > 0;
     }
-    default:
-        Q_UNREACHABLE();
     }
+    Q_UNREACHABLE();
+    return false;
 }
 
 /*! \internal