net: openthread: Remove waiting for DTR in openthread UART.

Uart driver for openthread have been waiting for host to start
communicating with coprocessor, during booting of the Zephyr and
by that blocking start os OS. There is no longer a need for
that since the stack will be soft rebooted after host connects
to coprocessor, removing the need to wait on host communication.

Signed-off-by: Przemyslaw Bida <przemyslaw.bida@nordicsemi.no>