Unfortunately, it is not possible to remove it.
"Reply STOP to opt out" is mandatory for all automated SMS considered as marketing messages. Due to the laws and regulations of most countries, you need to provide a way for your customers to unsubscribe from your SMSs in the future.
"Reply STOP to opt out" gives users the option to stop such messages in the future from the same sender if they consider it as spam. But since they subscribed in the first place to receive the notification of the product out of stock, most probably they won’t opt to stop the SMSs :)