mmc: core: Remove option not to use blk-mq

Remove config option MMC_MQ_DEFAULT and parameter mmc_use_blk_mq, so that
blk-mq must be used always.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>