avoid double choices reversing

Both yas-choose-value the yas-prompt-functions were reversing
values (cancelling each other out), instead just keep the choices in
order the whole time.
display-fn isn't actually used for filtering
yas-x-prompt: remove dead code
remove yas--x-pretty-prompt-templates
Merge pull request #466 from npostavs/no-reverse

Avoid double choices reversing