Responsive Image srcset Retaining Old Image Size

I installed a theme that registered a new 700px image size using add_image_size in functions.php. I decided I didn’t want that size and deregistered it (remove_image_size) then used plugins to delete all thumbnails and regenerate them, thus removing all of those 700px images. It does appear that the de-registration worked because 700px images are longer getting created. Problem: WordPress is still including links to those 700px images on old posts within the srcset of the responsive image code (700w), so they show up as broken images on certain resolution displays.

How to I get WordPress to totally forget about that old image size?

Thanks,
Mike