minor change

Author: nanfengpo

src/main/java/org/tron/core/witness/WitnessController.java

@@ -8,7 +8,6 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
-import java.util.Optional;
 import java.util.Set;
 import lombok.Getter;
 import lombok.Setter;