mmc: queue: delete mmc_req_is_special()

commit cdf8a6fb
"mmc: block: Introduce queue semantics"
deleted the last user of mmc_req_is_special() and it was
a horrible hack to classify requests as "special" or
"not special" to begin with, so delete the helper.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>