mmc: dw_mmc: Remove unused function for !CONFIG_OF

dw_mci_of_find_slot_node() is only used in dw_mci_of_get_slot_quirks()
if CONFIG_OF is defined, thus there is no need to have a !CONFIG_OF
version of it. Fixes the following compile warning with !CONFIG_OF:

  CC [M]  drivers/mmc/host/dw_mmc.o
drivers/mmc/host/dw_mmc.c:2223:28: warning: ‘dw_mci_of_find_slot_node’ defined but not used [-Wunused-function]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>