ARM: debug: remove extraneous DEBUG_HI3716_UART option

DEBUG_HI3716_UART was supposed to be renamed to DEBUG_HIX5HD2_UART, but
accidentally both got left in place, which results in a build error when
CONFIG_DEBUG_UART_PHYS is not set as it should be.

This removes the old symbol.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 12aae309 ("ARM: debug: Rename Hi3716 to HIX5HD2")
Acked-by: Wei Xu <xuwei5@hisilicon.com>