mmc: host: Retire MMC_GOLDFISH

Android Studio Emulator no longer uses this driver, so let's drop it.

Signed-off-by: Roman Kiryanov <rkir@google.com>
Link: https://lore.kernel.org/r/20210209190850.16647-1-rkir@google.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>