refactor: remove unused import

Author: javier-godoy

src/main/java/com/flowingcode/vaadin/addons/gridexporter/BaseStreamResourceWriter.java

@@ -38,8 +38,6 @@
 import java.util.List;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
-import org.apache.commons.lang3.tuple.ImmutablePair;
-import org.apache.commons.lang3.tuple.Pair;
 import org.apache.logging.log4j.util.Strings;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;