AP_Beacon: 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_Beacon/examples/AP_Marvelmind_test/AP_Marvelmind_test.cpp

@@ -16,7 +16,7 @@ void set_object_value_and_report(const void *object_pointer,
                       const char *name, float value);
 
 const AP_HAL::HAL& hal = AP_HAL::get_HAL();
-static AP_SerialManager serial_manager = AP_SerialManager::create();
+static AP_SerialManager serial_manager;
 AP_Beacon beacon{serial_manager};
 
 // try to set the object value but provide diagnostic if it failed