Revert "tty/serial: Add kgdb_nmi driver"

Revert "tty/serial: Add kgdb_nmi driver"

This reverts commit 0c57dfcc6c1d037243c2f8fbf62eab3633326ec0.

The functionality was supoosed to be used by a later patch in the
series that never landed [1]. Drop it.

NOTE: part of functionality was already reverted by commit
39d0be87438a ("serial: kgdb_nmi: Remove unused knock code"). Also note
that this revert is not a clean revert given code changes that have
happened in the meantime.

It's obvious that nobody is using this code since the two exposed
functions (kgdb_register_nmi_console() and
kgdb_unregister_nmi_console()) are both no-ops if
"arch_kgdb_ops.enable_nmi" is not defined. No architectures define it.

[1] https://lore.kernel.org/lkml/1348522080-32629-9-git-send-email-anton.vorontsov@linaro.org/

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: URL [1]
Link: https://lore.kernel.org/r/20250129082535.1.Ia095eac1ae357f87d23e7af2206741f5d40788f1@changeid
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>