nav2_rviz_plugins: Don't use non-existent slot

The definition of the slot was removed in commit
e6f500e ("nav2_rviz_plugins: Remove slots without
implementation (#4974)", 2025-03-10), because it had no
implementation. But we forgot to remove the reference to this slot,
because the compiler cannot detect it.

We remove the reference now. Without this, rviz shows warnings like:

    QObject::connect: No such slot nav2_rviz_plugins::CostmapCostTool::updateAutoDeactivate()
    QObject::connect:  (sender name:   'Single click')

Signed-off-by: Michal Sojka <michal.sojka@cvut.cz>

nav2_rviz_plugins/src/costmap_cost_tool.cpp

@@ -30,7 +30,7 @@ CostmapCostTool::CostmapCostTool()
   auto_deactivate_property_ = new rviz_common::properties::BoolProperty(
     "Single click", true,
     "Switch away from this tool after one click.",
-    getPropertyContainer(), SLOT(updateAutoDeactivate()), this);
+    getPropertyContainer(), nullptr, this);
 }
 
 CostmapCostTool::~CostmapCostTool() {}