hw/riscv: Remove all invalid uses of auto_create_sdcard=true

hw/riscv: Remove all invalid uses of auto_create_sdcard=true

MachineClass::auto_create_sdcard is only useful to automatically
create a SD card, attach a IF_SD block drive to it and plug the
card onto a SD bus. None of the RISCV machines modified by this
commit try to use the IF_SD interface.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20250204200934.65279-7-philmd@linaro.org>