perf: Remove redundant collection in Streaming Engine (#8645)

Author: tykus160

lib/media/streaming_engine.js

@@ -1027,22 +1027,17 @@ shaka.media.StreamingEngine = class {
      *              shaka.extern.Stream>}
      */
     const streamsByType = new Map();
-    /** @type {!Set<shaka.extern.Stream>} */
-    const streams = new Set();
 
     if (this.currentVariant_.audio) {
       streamsByType.set(ContentType.AUDIO, this.currentVariant_.audio);
-      streams.add(this.currentVariant_.audio);
     }
 
     if (this.currentVariant_.video) {
       streamsByType.set(ContentType.VIDEO, this.currentVariant_.video);
-      streams.add(this.currentVariant_.video);
     }
 
     if (this.currentTextStream_) {
       streamsByType.set(ContentType.TEXT, this.currentTextStream_);
-      streams.add(this.currentTextStream_);
     }
 
     // Init MediaSourceEngine.