DolphinQt: Make the mapping window clear button use ControllerEmu's clear functionality.

Source/Core/DolphinQt/Config/Mapping/MappingNumeric.cpp

@@ -28,13 +28,6 @@ void MappingNumeric::Connect()
           });
 }
 
-void MappingNumeric::Clear()
-{
-  m_setting->SetValue(m_setting->m_default_value);
-  m_parent->SaveSettings();
-  Update();
-}
-
 void MappingNumeric::Update()
 {
   setValue(m_setting->GetValue() * 100);