Fix compiler warning by removing Serializable interface implementation from ImmutableDocument test class

Author: jyemin

driver-core/src/test/unit/com/mongodb/client/ImmutableDocument.java

@@ -21,14 +21,13 @@
 import org.bson.codecs.configuration.CodecRegistry;
 import org.bson.conversions.Bson;
 
-import java.io.Serializable;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.Set;
 
-public final class ImmutableDocument implements Map<String, Object>, Serializable, Bson {
+public final class ImmutableDocument implements Map<String, Object>, Bson {
     private final Map<String, Object> immutableDocument;
 
     /**