mmc: core: Remove unnecessary check for the remove callback

For every bus_ops type the .remove callback always exist, thus there
are no need to check the existence of it, before we decide to call it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>