fix: do not prune all unused images

This would impact base images related to image building, as well as other images not managed by Dokku directly.

plugins/common/functions

@@ -649,9 +649,6 @@ docker_cleanup() {
     # delete danging images
     # shellcheck disable=SC2046
     "$DOCKER_BIN" rmi $("$DOCKER_BIN" images -f 'dangling=true' -q) &>/dev/null || true
-
-    # delete unused images
-    "$DOCKER_BIN" image prune --all &>/dev/null || true
   fi
 }