docs/manual: remove incorrect prerequites

The development files for zlib or netpbm are not needed, and neither
is python-xcbgen. None of these are present in the chroot used in the
autobuilders, and anyway if those would be needed, it would be a
Buildroot bug and not something to be mentionned in the documentation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

docs/manual/prerequisite.txt

@@ -74,10 +74,5 @@ development context (further details: refer to xref:download-infra[]).
 ** +qt4+ to use the 'xconfig' interface
 ** +glib2+, +gtk2+ and +glade2+ to use the 'gconfig' interface
 
-* Development libraries:
-** +zlib1+
-** +netpbm10+ (for +fbtest+)
-** +python-xcbgen+ (for +Matchbox+ on Debian based system)
-
 * Documentation generation tools:
 ** +asciidoc+