cmd: Remove mmc_spi command

The mmc_spi command was added to manually setup MMC over SPI bus
using command. This was required by the legacy non-DM MMC_SPI driver.

With DM based MMC_SPI driver in-place, we can now use all general
storge commands and mmc command for MMC over SPI bus hence we remove
the mmc_spi command all it's references.

Suggested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>