graph-depends: remove support for "unknown" packages

The "unknown" packages mechanism was used to render packages that did
not implement the make <pkg>-show-depends target, i.e the packages
that were not yet converted to one of the package infrastructures.

Since now all packages have been converted, we can remove this
"unknown" packages feature.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>