Revert "DocBook: Avoid building man pages repeatedly and inconsistently"

This reverts commit b44158b1

.  This commit
introduced warnings and possibly inconsistent results into the doc build
process.  The goal is good but it will need to be achieved another way.

Reported-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>