Unfortunately there is no way to strip the HTML. In nopCommerce version 3.4 there is a setting which if enabled the product short description can be used for meta description which is much more advised as the short description is a plain text field and there is a size limitation on it. It is better to use it as the meta description tag length should be no more of 156 chars for optimal results. Is there any specific reason why you should want to use the product full description for a meta description.