Remove the EnableSignatureChecks setting

Since Dolphin can do NUS downloads over plain HTTP, we really don't
want people to be able to silently disable signature verification
indefinitely. Removing the setting shouldn't have any significant
negative impact now that signature verification always is disabled
when installing WAD files.

Source/Core/Core/ConfigManager.h

@@ -166,8 +166,6 @@ struct SConfig
   std::set<std::pair<u16, u16>> m_usb_passthrough_devices;
   bool IsUSBDeviceWhitelisted(std::pair<u16, u16> vid_pid) const;
 
-  bool m_enable_signature_checks = true;
-
   // Fifo Player related settings
   bool bLoopFifoReplay = true;