Blackfin: remove unused is_in_rom()

The function is not used anywhere in the whole tree (anymore), so remove
it.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bob Liu <lliubbo@gmail.com>