How to remove non-launchpad-ppa repository from apt update list?
Solution 1:
Run grep -r realsense /etc/apt to find any files in /etc/apt that include that source. For example, it could be in /etc/apt/sources.list.
apt only processes source list files that end in .list. Any files named .list.save, .list.distUpgrade, and so on are just backups that apt doesn't use.