Nicla_System: fix warning by removing unused variable (#552)

I removed the char "response" in the method readByte as it was unused and it would just give a compiler warning

Author: alessandromrc

libraries/Nicla_System/src/pmic_driver.cpp

@@ -25,7 +25,6 @@ uint8_t BQ25120A::readByte(uint8_t address, uint8_t subAddress)
 {
   digitalWrite(p25, HIGH);
   nicla::i2c_mutex.lock();
-  char response = 0xFF;
   Wire1.beginTransmission(address);
   Wire1.write(subAddress);
   Wire1.endTransmission(false);
@@ -37,4 +36,4 @@ uint8_t BQ25120A::readByte(uint8_t address, uint8_t subAddress)
   nicla::i2c_mutex.unlock();
   digitalWrite(p25, LOW);
   return ret;
-}
+}