drivers: sensor: voltage_divider: revert patch

A patch was added to the voltage divider to handle an erroneous
negative voltage reading for the nrfx_saadc which could return
negative voltages even though the ADC mode was single ended.

This has now been patched in the ADC driver.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>