Blackfin arch: remove dump_thread()

The only user is the a.out support.

It was therefore removed prior to the blackfin merge from all
architectures not supporting a.out.

Currently, Blackfin doesn't suppport a.out.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>