configs: remove gnublin_defconfig

This defconfig tries to fetch its kernel source code and U-Boot source
code from gitorious.org, which is not longer available. Therefore,
this defconfig has been failing to build since quite some time. Since
there's no obvious other place to grab the kernel and u-boot source
code, and the defconfig was not updated since a long time, let's
simply get rid of it.

[Peter: also drop README]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>