env: mmc: Clean up env_mmc_load() ifdeffery

env: mmc: Clean up env_mmc_load() ifdeffery

Rename the variants of env_mmc_load() for redundant and non-redundant
environment to env_mmc_load_redundant() and env_mmc_load_singular()
respectively and convert the env_mmc_load() implementation to use of
if (IS_ENABLED(...)). As a result, drop __maybe_unused from
mmc_env_is_redundant_in_both_boot_hwparts().

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>