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>