kallsyms: remove left-over Blackfin code

These symbols were added by commit 028f0426 ("kallsyms: support
kernel symbols in Blackfin on-chip memory") for Blackfin.

The Blackfin support was removed by commit 4ba66a97 ("arch: remove
blackfin port").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>