Refactor: Make AbstractCheck non-validating

CityDoctorParent/CityDoctorModel/src/main/java/de/hft/stuttgart/citydoctor2/check/AbstractCheck.java

@@ -50,4 +50,9 @@ public class AbstractCheck extends Check {
 		return null;
 	}
 
+	@Override
+	public boolean isValidator() {
+		return false;
+	}
+
 }