Based on my experience, it is possible to remove CNAME/TXT or HTML file verification once the domain has been verified. There are few sales threads with expired domains in marketplace and the thread owner still hold its ownership (even after CNAME and file verification is no longer exist). It means once domain has been verified, it will stay verified under first owner*. While it is easy to remove CNAME and HTML file verification after the domain has been verified, I think it is better not to remove it. In my opinion, each CNAME and file verification is unique and safe. Having CNAME/TXT record will not reduce domain's value. Keeping File Verification in root folder would not slowing down website nor take too much space. There is no security flaw having CNAME and file verification. It has been common procedure to verify domain ownership. * The rules maybe changed in future. A new feature could be added in future such as re-checking ownership once it re-listed or sales thread being modified. So, it is better to keep domain ownership verification if the domain/website is still available for sale. If the domain is used to provide service such as hosting, etc. I encourage to keep the ownership verification in root folder or DNS record.
You can remove it if you aren't going to use it for anything else. Later if you want to relist or use other services that require domain verification (Digital Point Ads, etc.) you would need to readd it. It doesn't hurt anything, but it can be removed if you aren't going to do anything with it in the future.