agent: remove pgrep -U arg, broken under zsh

Author: msimerson

agent.sh

@@ -77,7 +77,7 @@ main()
  then
  echo "ssh-agent socket is stale"
  kill "${_agent_pid}" # kill ssh-agent
- killall pgrep ssh-agent -U "$USER"
+ killall pgrep ssh-agent
  cleanup_stale_agent
  start_ssh_agent
  return