mmc: core: Remove support for MMC_CAP2_NO_SLEEP_CMD

There are no active users of this host capability. The primary reason
for adding this cap was due to a bug in ux500 boot loader code, which
is not a relevant issue any more. So, let's remove it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>