Disable Drush backend messages by default
Uninstall cleanly that version of drush as 5.0 is very old now. Uninstall using pear if you installed that way, using Composer, or manually checking which drush and remove symlinks, drush folder, backup and then remove ~/.drush.

Look at the Drush install page to see what drush version you need for Drupal version you are working with. Then install drush using Composer which is the recommended way.

This message should be gone then. Good luck!