avoid double choices reversing

This isn't ready for merging yet, I've only tested with `yas-choose-value` so far. I wanted to ask about the `display-fn` parameter: it seems it's only used from `yas--prompt-for-template` `yas--prompt-for-table` with values `#'yas--template-name` and `#'yas--table-name`, respectively. Could there ever be nameless templates or tables? Because removing the (apparently unused) filtering aspect of `display-fn` would simplify things further.

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As I mentioned in https://github.com/capitaomorte/yasnippet/pull/453#issuecomment-36445727, remove the double reversal in `yas-choose-value` and the `yas-prompt-functions`. Actually, I noticed `yas-no-prompt` wasn't reversing so before applying this patch:
```el
(let ((yas-prompt-functions '(yas-no-prompt)))
  (yas-choose-value "one" "two" "three")) ; => "three"
```

After applying it gives "one" as expected.