Revert "imx: Introduce CONFIG_SPL_FORCE_MMC_BOOT to force MMC boot on falcon mode"

Revert "imx: Introduce CONFIG_SPL_FORCE_MMC_BOOT to force MMC boot on falcon mode"

The CONFIG_SPL_FORCE_MMC_BOOT config flag is not needed as its behavior
is the correct one in all cases;  using spl_boot_device() instead of the
boot_device parameter will lead to inconsistency issues, for example,
when a board_boot_order() is defined.  In fact, this is the reason the
parameter was introduced in the first place, in commit 2b1cdafa9fdd
("common: Pass the boot device into spl_boot_mode()").

This reverts commit 04c4b22243498eafb9339ac1eea753bdf9db9858.

Link: https://lists.denx.de/pipermail/u-boot/2020-April/405979.html
Signed-off-by: Harald Seiler <hws@denx.de>