drivers: clock_control: nrf: hfxo: Remove redundad code

There were redundant code in full_irq_lock(), full_irq_unlock()
functions that supposed to be used when ZLI IRQs are disabled.
These functions are compiled in only when CONFIG_ZERO_LATENCY_IRQS
is set, hence the non-ZLI execution path was never included
in final binaries.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>