manual: developer-guide.txt: cleanup non-existing source

[Thomas: Added ThomasDS Acked-by, which was given on an earlier
version of the same patch.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>