Remove deprecated MapWriter.append

Author: dsmiley

solr/solrj/src/java/org/apache/solr/common/MapWriter.java

@@ -74,15 +74,6 @@ public MapWriter.EntryWriter put(CharSequence k, Object v) {
     writer.endObject();
   }
 
-  @Deprecated
-  default MapWriter append(MapWriter another) {
-    MapWriter m = this;
-    return ew -> {
-      m.writeMap(ew);
-      another.writeMap(ew);
-    };
-  }
-
   /**
    * An interface to push one entry at a time to the output. The order of the keys is not defined,
    * but we assume they are distinct -- don't call {@code put} more than once for the same key.