zephyr: serial: Remove unnecessary call to irq_unlock

The following commit in Zephyr removed the need to unlock interrupts
when booting in single-threaded mode:
https://github.com/zephyrproject-rtos/zephyr/commit/3b89cf173b6b0085c84a09fd97b42e08a25ae4b3



Remove the now obsolete lines.

Fixes #302.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>