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>