ui/input-legacy.c: remove unused legacy qemu_add_kbd_event_handler() function

ui/input-legacy.c: remove unused legacy qemu_add_kbd_event_handler() function

Since the last keyboard device has now been converted over to use
qemu_input_handler_register(), the legacy qemu_add_kbd_event_handler() function
is now unused and can be removed.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20241106120928.242443-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <thuth@redhat.com>