ARM: imx: drop CONFIG_MMC_UNSAFE_RESUME from defconfig

Since 2501c917 (mmc: core: Use MMC_UNSAFE_RESUME as default behavior),
CONFIG_MMC_UNSAFE_RESUME is not available anymore.  Drop it from imx
defconfig.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>