Delete orphaned property default for exit_code which is not exposed any more.

core/core_bind.cpp

@@ -623,7 +623,6 @@ void OS::_bind_methods() {
 
 	// Those default values need to be specified for the docs generator,
 	// to avoid using values from the documentation writer's own OS instance.
-	ADD_PROPERTY_DEFAULT("exit_code", 0);
 	ADD_PROPERTY_DEFAULT("low_processor_usage_mode", false);
 	ADD_PROPERTY_DEFAULT("low_processor_usage_mode_sleep_usec", 6900);