mmc: remove SDHCI SPEAR

As the file spear_sdhci.c file is already removed, delete the associated
configuration CONFIG_MMC_SDHCI_SPEAR.

Fixes: c942fc92 ("mmc: spear: remove the entire spear_sdhci.c file")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>