kdb: Remove unused flags stack

kdb_restore_flags() and kdb_save_flags() were added in 2010 by
commit 5d5314d6 ("kdb: core for kgdb back end (1 of 2)")
but have remained unused.

Remove them, and their associated storage.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20250112012049.319515-1-linux@treblig.org


Signed-off-by: Daniel Thompson (RISCstar) <danielt@kernel.org>