Update load-cookie documentation

[dgirardi]

https://github.com/prebid/user-sync/pull/1 is a reimplementation of load-cookie.html and load-cookie-with-consent.html. The latter is there only for backwards compatibility and no longer needed; this updates the docs to reflect it.

However, note that it's safe to direct everyone to the new load-cookie.html only after all PBS hosting companies have switched to the new version.

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