Revert "drivers: ieee802154: nrf: cache radio channel"

This reverts commit 780b1285.
"drivers: ieee802154: nrf: cache radio channel"

Implementation affected RCP devices in openthread as MAC layer
does not call `Receive()` functions after transmit is done.

Additionally, after sending a frame to a new channel (for example
while discovery operation), radio switches to RX state immediately
after TX, but continues to listen on old channel for about 5ms,
until MAC layer calls `Receive` operation, forcing to change the
channel.

Signed-off-by: Maciej Baczmanski <maciej.baczmanski@nordicsemi.no>