I had to uninstall Avast because of a conflict with my system. I used the Avast removal tool, but I just went thru a malware removal process where a Tech helped, we did a system Info scan and there are still Avast folders or files on my system.
What version of avast was you using ? As for leftovers, i think they are from an older version that was unninstalled without the removal tool, or you just upgraded over the top of it.
Alternatively you just can delete them manually.
Also only “credible” system cleaner…FREE version works fine.
After you’ve used the uninstaller and AvastClear you can run this and get old crud out. https://www.piriform.com/CCLEANER