Revert "Workaround man page output for long PREFIX dirs"

This reverts commit 83d9f436.

man is still unable to wrap some long lines, so we simply revert this patch
(and filter out the specific message in autotools/check-man).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>