tests: adc_simple: Change ADC_DEBUG=y to SYS_LOG_ADC_LEVEL=4

The ADC_DEBUG Kconfig symbol was removed in commit a679f329
("sys_log: replace old debug macros at ADC driver"), which introduced
SYS_LOG_ADC_LEVEL and switched to using SYS_LOG_DBG() for debug
messages.

Remove the ADC_DEBUG=y assignment and set SYS_LOG_ADC_LEVEL=4 so that
the debug messages still show up.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>