It was removed because it was not very helpful. The default search range for that box was 1 week, and people were very unlikely to find what they were looking for by using it. Most of the time the search from that box resulted in the "no posts found" screen, and the user was unable to search again right away, since our forums, like most forums, have limits on how often a user can perform a search (to prevent malicious attempts to bring down the forums).
The Search link at the top provides a much more useful search function that allows you to set the date range (up to 2 years) and is a much better choice to use.
