How do I remove the appended string from image and Javascript URLs?
The strings are to ensure that the files are not cached by proxies or intermediary caches when Drupal rebuilds its own caches. Whenever Drupal's cache is cleared and regenerated these strings will change. These are 'cache buster' strings.

Check this for an explanation of what is cache busting - https://www.adopsinsider.com/ad-ops-basics/what-is-a-cache-buster-and-how-does-it-work/