drivers: uart_nrfx_uarte: Revert workaround for bytes dropping

This effectively reverts the following three commits:
- 0f9f1884
- 68124410
- 326f7bd4
and also the changes that got copied to the nrf5340_audio_dk_nrf5340
board.

The workaround brings more harm than good. It already required many
tweaks in various tests to make them pass (because it introduced
a significant overhead in processing of the console UART interrupt)
and now it makes the I2S driver tests to fail. It's not reasonable
to add more tweaks in Zephyr tests just to keep this workaround in
the tree. Instead the root cause should be fixed (if the original
problem still occurs).

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>