The function kdb_position_cursor() takes in a "prompt" parameter but
never uses it. This doesn't _really_ matter since all current callers
of the function pass the same value and it's a global variable, but
it's a bit ugly. Let's clean it up.

Found by code inspection. This patch is expected to functionally be a
no-op.

Fixes: 09b35989 ("kdb: Use format-strings rather than '\0' injection in kdb_read()")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20240528071144.1.I0feb49839c6b6f4f2c4bf34764f5e95de3f55a66@changeidSigned-off-by: Daniel Thompson <daniel.thompson@linaro.org>