finagle-doc: Remove advice for very old finagle from the FAQ
finagle-doc: Remove advice for very old finagle from the FAQ

Problem / Solution

6.39.0 is a truly ancient Finagle, and so we don't receive questions about this
frequently anymore.  We should remove the advice for pre-6.39.0 from the FAQ.

JIRA Issues: CSL-7721

Differential Revision: https://phabricator.twitter.biz/D396801

twitter/finagle,twitter/finagle,twitter/finagle,twitter/finagle,twitter/finagle