Remove stop_docker from CI script

Update our CI script to no longer attempt to stop docker. We don't
call it anyway and it will be one less moving part trying to diagnose
why `dockerd` sometimes terminates early.

See gh-18127

Author: philwebb

ci/images/docker-lib.sh

@@ -93,20 +93,9 @@ start_docker() {
 
   export server_args DOCKER_LOG_FILE
   declare -fx try_start
-  trap stop_docker EXIT
 
   if ! timeout ${STARTUP_TIMEOUT} bash -ce 'while true; do try_start && break; done'; then
     echo Docker failed to start within ${STARTUP_TIMEOUT} seconds.
     return 1
   fi
 }
-
-stop_docker() {
-  local pid=$(cat /tmp/docker.pid)
-  if [ -z "$pid" ]; then
-    return 0
-  fi
-
-  kill -TERM $pid
-}
-