arch: blackfin: kernel: kgdb: Remove unused function

Remove the function kgdb_post_primary_code() that is not used anywhere.

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Acked-by: Sonic Zhang <sonic.zhang@analog.com>