serial: Remove obsolete CONS_INDEX_n Kconfig options

These were only ever implied by sunxi platforms, and that usage has
been removed. Current practice is to specify CONFIG_CONS_INDEX in each
board's defconfig.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>