arch: remove blackfin port

arch: remove blackfin port

The Analog Devices Blackfin port was added in 2007 and was rather
active for a while, but all work on it has come to a standstill
over time, as Analog have changed their product line-up.

Aaron Wu confirmed that the architecture port is no longer relevant,
and multiple people suggested removing blackfin independently because
of some of its oddities like a non-working SMP port, and the amount of
duplication between the chip variants, which cause extra work when
doing cross-architecture changes.

Link: https://docs.blackfin.uclinux.org/
Acked-by: Aaron Wu <[email protected]>
Acked-by: Bryan Wu <[email protected]>
Cc: Steven Miao <[email protected]>
Cc: Mike Frysinger <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>