how do i remove robots txt file from website shopify properly?

You can't delete it on Shopify, every store has one. You can edit it though: Online Store > Themes > Edit code, add a new template and choose robots, which creates robots.txt.liquid. Change the rules in there. I'd be careful about stripping it out completely. The defaults block cart, checkout and the filtered/sorted collection URLs, and on 300 products those filter combinations can be a lot of junk for Google to crawl. Usually you only want to remove the one rule that's in your way. Also robots.txt only controls crawling, it doesn't remove anything from the index. If you're trying to get pages out of Google, you need noindex instead, and the page has to stay crawlable so Google can see it. That might be why you saw no difference.