mtd: remove SPEAr flash driver st_smi

Remove the driver st_smic.c used in SPEAr products and the associated
config CONFIG_ST_SMI; this driver is no more used in U-Boot after the
commit 570c3dcf ("arm: Remove spear600 boards and the rest of SPEAr
support").

Fixes: 570c3dcf ("arm: Remove spear600 boards and the rest of SPEAr support")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>