Some (but not all?) of the kdb backtrace paths would cause the
kdb_current_task and kdb_current_regs to remain changed.  As discussed
in a review of a previous patch [1], this doesn't seem intuitive, so
let's fix that.

...but, it turns out that there's actually no longer any reason to set
the current task / current regs while backtracing anymore anyway.  As
of commit 2277b492 ("kdb: Fix stack crawling on 'running' CPUs
that aren't the master") if we're backtracing on a task running on a
CPU we ask that CPU to do the backtrace itself.  Linux can do that
without anything fancy.  If we're doing backtrace on a sleeping task
we can also do that fine without updating globals.  So this patch
mostly just turns into deleting a bunch of code.

[1] https://lore.kernel.org/r/20191010150735.dhrj3pbjgmjrdpwr@holly.lanSigned-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191109111624.4.Ibc3d982bbeb9e46872d43973ba808cd4c79537c7@changeidSigned-off-by: Daniel Thompson <daniel.thompson@linaro.org>