Drop explicit version check for flex

Summary: I don't really think this is needed anymore. Although there is ~zero cost in keeping this check around, I wouldn't expect anyone to have such an old version of flex installed. According to the flex mailing list, v2.5.33 was released in 2006.

Test Plan: Ran make scanner

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11628