refactor(st): remove unused io_mmc driver

This driver was used when STM32MP_USE_STM32IMAGE was enabled. This flag
is now removed, so the ST io_mmc driver can now be removed.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I3c1280dec8926b921534c81e143e86cfe6d4ee0d