samples: net: cellular_modem: improve nRF9160 DK configurations

- Async UART would start receiving some data incorrectly
  without HW RX byte counting at baud rates above 115200.

- Increase the UART buffer sizes to be able to receive
  whole UART TXs of 1500 bytes sent by SLM at once.

- Remove the Kconfig items whose definition is now redundant.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>