Removed Benzene from MQ2 sensor

examples/MQ-2/MQ-2.ino

@@ -63,7 +63,6 @@ void setup() {
   CO =  MQ2.readSensor("CO"); // Return CO concentration
   Alcohol =  MQ2.readSensor("Alcohol"); // Return Alcohol concentration
   Propane =  MQ2.readSensor("Propane"); // Return Propane concentration
-  Benzene =  MQ2.readSensor("Benzene"); // Return Benzene concentration
   
   Serial.println("***************************");
   Serial.println("Lectures for MQ-2");
@@ -74,6 +73,5 @@ void setup() {
   Serial.print("CO: ");Serial.print(CO,2);Serial.println(" ppm");
   Serial.print("Alcohol: ");Serial.print(Alcohol,2);Serial.println(" ppm");
   Serial.print("Propane: ");Serial.print(Propane,2);Serial.println(" ppm");
-  Serial.print("Benzene: ");Serial.print(Benzene,2);Serial.println(" ppm");
   Serial.println("***************************");
 }

src/MQUnifiedsensor.cpp

@@ -169,12 +169,6 @@ void MQUnifiedsensor::setSensorCharacteristics(String nameLectureRequeired, bool
         _nameLectureRequeired = "Propane";
         _a = MQ2_Propane_a;
         _b = MQ2_Propane_b;
-      }   
-      else if(nameLectureRequeired == "Benzene")
-      {
-        _nameLectureRequeired = "Benzene";
-        _a = MQ2_Benzene_a;
-        _b = MQ2_Benzene_b;
       }    
     }
     else if(_type == 3)