mmc: omap_hsmmc: remove unused get_context_loss_count callback

trigger of this callback has been removed in 0a82e06e

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Andreas Fenkart <afenkart@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>