tests uart/uart_async_api: Improve filter

These tests were filtering by CONFIG_UART_CONSOLE,
but that does not seem a strick requirement.
It seems the requirement would just be CONFIG_SERIAL,
but that is implicit if CONFIG_SERIAL_SUPPORT_ASYNC,
which is required for all these tests, so let's
just remove CONFIG_UART_CONSOLE.

(The simulated nrf5x boards do not set CONFIG_UART_CONSOLE
by default, as by default the console is routed to the
process stdout instead)

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>