Blackfin: Remove

The architecture is currently unmaintained, remove.

Cc: Benjamin Matthews <mben12@gmail.com>
Cc: Chong Huang <chuang@ucrobotics.com>
Cc: Dimitar Penev <dpn@switchfin.org>
Cc: Haitao Zhang <hzhang@ucrobotics.com>
Cc: I-SYST Micromodule <support@i-syst.com>
Cc: M.Hasewinkel (MHA) <info@ssv-embedded.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Martin Strubel <strubel@section5.ch>
Cc: Peter Meerwald <devel@bct-electronic.com>
Cc: Sonic Zhang <sonic.adi@gmail.com>
Cc: Valentin Yakovenkov <yakovenkov@niistt.ru>
Cc: Wojtek Skulski <info@skutek.com>
Cc: Wojtek Skulski <skulski@pas.rochester.edu>
Signed-off-by: Tom Rini <trini@konsulko.com>