mmc: sdhci-pci: remove outdated declaration

The function was removed half a year ago, so this declaration can go,
too.

Fixes: 51ced59c ("mmc: sdhci-pci: Use ACPI DSM to get driver strength for some Intel devices")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>