contrib/flex: Drop local __dead2 patch

Upstream flex has added a yynoreturn macro, so this diff is no longer
needed. Partially reverts r181269.

Reviewed By: jkim
Differential Revision: https://reviews.freebsd.org/D29679

(cherry picked from commit d37f81e35b6e8f114b04315e06bab2cf0c03946c)