Revert "imx: defconfig: Enable CONFIG_SPL_FORCE_MMC_BOOT"

CONFIG_SPL_FORCE_MMC_BOOT was removed in a previous patch as its
behavior is the correct one in all cases.  Remove all uses of it from
defconfigs.

This reverts commit 3201e5b4 and removes
CONFIG_SPL_FORCE_MMC_BOOT from the imx28_xea defconfig.

Signed-off-by: Harald Seiler <hws@denx.de>