AP_Relay: removed create() method for objects

See discussion here:

  https://github.com/ArduPilot/ardupilot/issues/7331

we were getting some uninitialised variables. While it only showed up in
AP_SbusOut, it means we can't be sure it won't happen on other objects,
so safest to remove the approach

Thanks to assistance from Lucas, Peter and Francisco

libraries/AP_Relay/AP_Relay.h

@@ -17,9 +17,7 @@
 /// @brief	Class to manage the ArduPilot relay
 class AP_Relay {
 public:
-    static AP_Relay create() { return AP_Relay{}; }
-
-    constexpr AP_Relay(AP_Relay &&other) = default;
+    AP_Relay();
 
     /* Do not allow copies */
     AP_Relay(const AP_Relay &other) = delete;
@@ -43,8 +41,6 @@ public:
     static const struct AP_Param::GroupInfo        var_info[];
 
 private:
-    AP_Relay();
-
     AP_Int8 _pin[AP_RELAY_NUM_RELAYS];
     AP_Int8 _default;
 };