include/configs: drop default definitions of CONFIG_SYS_MAXARGS

Now that include/config_fallbacks.h define a sane fallback for
CONFIG_SYS_MAXARGS, we can drop the definition of this constant in all
configurations that were using the default value.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>