Revert potentially breaking change

I expected this class to be located in ``youtube`` as it mostly contains YT specific playlist types, but this assumption was wrong.

See also: https://github.com/TeamNewPipe/NewPipeExtractor/pull/1277/files/d9caa90d239571dfa394923ab7b7c35004454a06#r1955161108

Author: litetex

extractor/src/main/java/org/schabi/newpipe/extractor/playlist/PlaylistInfo.java

@@ -13,11 +13,12 @@
 import org.schabi.newpipe.extractor.stream.StreamInfoItem;
 import org.schabi.newpipe.extractor.utils.ExtractorHelper;
 
-import javax.annotation.Nonnull;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
 
+import javax.annotation.Nonnull;
+
 public final class PlaylistInfo extends ListInfo<StreamInfoItem> {
 
     /**
@@ -42,6 +43,16 @@ public enum PlaylistType {
          */
         MIX_MUSIC,
 
+        /**
+         * A mix made only of streams from (or related to) the same channel, for example YouTube
+         * channel mixes
+         *
+         * @deprecated There is currently no service that implements this.
+         * YouTube removed all playlists with this type around 2024-06
+         */
+        @Deprecated
+        MIX_CHANNEL,
+
         /**
          * A mix made only of streams related to a particular (musical) genre, for example YouTube
          * genre mixes