led: remove support for red LED in legacy API

led: remove support for red LED in legacy API

To the exception of red_led_on in the arm-specific assembly code, all
code interacting with the red status LED was guarded by the
CONFIG_LED_STATUS_RED symbol, which is enabled in none of the upstream
defconfigs.

Since the last board which overrode the weak red_led_on function got
migrated to the new LED mechanism, there's also no user of the
arm-specific assembly code anymore, therefore it can be removed along
the other unreachable code sections.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Heiko Schocher <hs@nabladev.com>