hw/sd: Remove unused SDState::enable

hw/sd: Remove unused SDState::enable

Now that sd_enable() has been removed, SD::enable is set to true in
sd_instance_init() and then never changed. So we can remove it.

Note that the VMSTATE_UNUSED() size argument should be '1', not
'sizeof(bool)', as noted in the CAUTION comment in vmstate.h.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250128104519.3981448-12-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>