serial: kgdb_nmi: Remove unused knock code

serial: kgdb_nmi: Remove unused knock code

kgdb_nmi_poll_knock() has been unused since it was added in 2013 in
commit 0c57dfcc6c1d ("tty/serial: Add kgdb_nmi driver")

Remove it, the static helpers, and module parameters it used.

(The comment explaining why it might be used sounds sensible, but
it's never been wired up, perhaps it's worth doing somewhere?)

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://lore.kernel.org/r/20250112135759.105541-1-linux@treblig.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>