MINOR: Remove line with addAll since collection passed into constructor call. (#7807)

Reviewers: Guozhang Wang <wangguoz@gmail.com>, Sophie Blee-Goldman <sophie@confluent.io>

streams/src/main/java/org/apache/kafka/streams/kstream/internals/AbstractStream.java

@@ -87,7 +87,6 @@ public abstract class AbstractStream<K, V> {
 
     Set<String> ensureCopartitionWith(final Collection<? extends AbstractStream<K, ?>> otherStreams) {
         final Set<String> allSourceNodes = new HashSet<>(sourceNodes);
-        allSourceNodes.addAll(sourceNodes);
         for (final AbstractStream<K, ?> other: otherStreams) {
             allSourceNodes.addAll(other.sourceNodes);
         }