Revert "Use @greggles version of Drush Alias for Fish prompt, much faster."

This reverts commit f46e4a3cbdcac78a294d53a14107d3b66d3524cb.

Conflicts:
	themes/agnoster/fish_prompt.fish

Reverted because the /tmp/drush... folder doesn't always seem to be
there. Using drush site-get is slower but seems to work better. It is
slower because it bootstraps drupal with a drush eval.