config/raspberrypi: remove non-DT defconfig

The RPi is slowly but surely dropping support for booting without a DTB,
so just ditch our old defconfig now that we have up-to-date DT-enabled
defconfigs for all RPi models (save for the compute module).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Floris Bos <bos@je-eigen-domein.nl>
Cc: Pascal de Bruijn <pmjdebruijn@pcode.nl>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>