MAINTAINERS: mmc: Remove the SDHCI-OF section

Anton told me that he isn't maintaing the SDHCI-OF parts anymore, so
let's remove him from this section to avoid confusion.

Morover, since the SDHCI-OF section overlaps with the SDHCI DRIVER
section, let's just remove it completely.

Cc: Anton Vorontsov <anton@enomsg.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>