Thank you for reaching out.
As you wrote, the tooltip uses the alt text that is added in WordPress. Thats not a feature of Recent Posts Widget With Thumbnails. There is no option to remove only part of the alt text with the plugin.
You have to remove the number from the alternitive text of the image. Thats the only solution I can offer.
By the way. Removing the alt text is a bad idea in terms of search engines.
I’m sorry I can’t give a better answer.