InputCommon: Eliminate some duplicated button threshold logic.

Source/Core/InputCommon/ControllerEmu/ControlGroup/Slider.cpp

@@ -32,7 +32,7 @@ Slider::Slider(const std::string& name_) : Slider(name_, name_)
 Slider::StateData Slider::GetState()
 {
   const ControlState deadzone = m_deadzone_setting.GetValue() / 100;
-  const ControlState state = controls[1]->control_ref->State() - controls[0]->control_ref->State();
+  const ControlState state = controls[1]->GetState() - controls[0]->GetState();
 
   return {std::clamp(ApplyDeadzone(state, deadzone), -1.0, 1.0)};
 }