kgdb: Remove unused DCPU_SSTEP definition

From doing a 'git log --patch kernel/debug', it looks as if DCPU_SSTEP
has never been used.  Presumably it used to be used back when kgdb was
out of tree and nobody thought to delete the definition when the usage
went away.  Delete.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Jason Wessel <jason.wessel@windriver.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>