i2c: adi_i2c: remove left-over Blackfin I2C driver

This driver was used by Blackfin boards, but Blackfin support is
gone.  There is no user of this driver.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Heiko Schocher <hs@denx.de>