Revert "kernel/debug: Mask KGDB NMI upon entry"

This reverts commit 5a14fead.

No architectures ever implemented `enable_nmi` since the later patches
in the series adding it never landed. It's been a long time. Drop it.

NOTE: this is not a clean revert due to changes in the file in the
meantime.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20250129082535.3.I2254953cd852f31f354456689d68b2d910de3fbe@changeid


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>