AP_ADC: correct compilter warnings

../../libraries/AP_ADC/AP_ADC_ADS1115.h:34:25: warning: private field
'_last_update_timestamp' is not used [-Wunused-private-field]
    uint32_t            _last_update_timestamp;
                        ^
1 warning generated.

libraries/AP_ADC/AP_ADC_ADS1115.h

@@ -31,7 +31,6 @@ private:
 
     AP_HAL::OwnPtr<AP_HAL::I2CDevice> _dev;
 
-    uint32_t            _last_update_timestamp;
     uint16_t            _gain;
     int                 _channel_to_read;
     adc_report_s        *_samples;