MIPS: kdb: Remove old workaround for backtracing on other CPUs

As of commit 2277b492

 ("kdb: Fix stack crawling on 'running' CPUs
that aren't the master") we no longer need any special case for doing
stack dumps on CPUs that are not the kdb master.  Let's remove.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191109111623.1.I30a0cac4d9880040c8d41495bd9a567fe3e24989@changeid


Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>