mmc: core: remove dead function mmc_try_claim_host

cscope says there are no callers for mmc_try_claim_host in the kernel.
No reason to keep it.

Signed-off-by: Grant Grundler <grundler@chromium.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>