mmc: core: Drop redundant member in struct mmc host

The Kconfig option to use the blk-mq support was removed in commit
1bec43a3 ("mmc: core: Remove option not to use blk-mq"), but forgot to
remove the use_blk_mq member in the struct mmc_host, let's fix it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20210202101924.69970-1-ulf.hansson@linaro.org