Fix man/Makefile for forks

The chocolate-{server,serup}.6 manpages are static and not auto-generated, so they do not get renamed when the @PROGRAM_PREFIX@ variable changes. This breaks the Make rules for forks, which will have to create their own server and setup manpages anyway. This reverts part of commit 08ad5553 by myself which introduced the issue.

Fixes https://github.com/fabiangreffrath/crispy-doom/issues/2