util: remove obsolete SETVBUF_REVERSED

util: remove obsolete SETVBUF_REVERSED

Commit 8af5762909714367c1cc764b3f491c06c2bcd5d0 "Clean up some
obsolete Autoconf code" removed the obsolete autoconf check
AC_FUNC_SETVBUF_REVERSED and one use of the results, but
overlooked another instance; remove it.

Change-Id: Id62a2a96b911c0d16d51d8cce0966ae3736bde87
Reviewed-on: https://gerrit.openafs.org/11718
Reviewed-by: Chas Williams <3chas3@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>