Remove superfluous import

Author: TomyLobo

worldedit-core/src/main/java/com/sk89q/worldedit/EditSession.java

@@ -136,7 +136,6 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicInteger;
-import java.util.stream.Collectors;
 import javax.annotation.Nullable;
 
 import static com.google.common.base.Preconditions.checkArgument;