tests: drivers: uart: uart_mix_fifo_poll: Clean up testcase.yaml

Clean up after automatic merge. Some test configurations got removed
and multiple lines enabling test for nrf54l09pdk/nrf54l09/cpuapp
target remained at the bottom of the file.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>