fix(config): remove enabled config option duplication

Author: ArturMoczulski

rollbar-logback/src/main/java/com/rollbar/logback/RollbarAppender.java

@@ -50,8 +50,6 @@ public class RollbarAppender extends AppenderBase<ILoggingEvent> {
   private String framework;
 
   private String language;
-  
-  private boolean enabled;
 
   private String platform;