chore: Reduce same conditions in StreamingEngine (#8709)

lib/media/streaming_engine.js

@@ -2468,8 +2468,6 @@ shaka.media.StreamingEngine = class {
         stream.closedCaptions.size > 0;
 
     if (this.config_.shouldFixTimestampOffset) {
-      let parser;
-
       const isMP4 = stream.mimeType == 'video/mp4' ||
               stream.mimeType == 'audio/mp4';
       let timescale = null;
@@ -2481,11 +2479,7 @@ shaka.media.StreamingEngine = class {
           this.manifest_.type == shaka.media.ManifestParser.DASH;
 
       if (shouldFixTimestampOffset) {
-        parser = new shaka.util.Mp4Parser();
-      }
-
-      if (shouldFixTimestampOffset) {
-        parser
+        new shaka.util.Mp4Parser()
             .box('moof', shaka.util.Mp4Parser.children)
             .box('traf', shaka.util.Mp4Parser.children)
             .fullBox('tfdt', async (box) => {
@@ -2519,11 +2513,8 @@ shaka.media.StreamingEngine = class {
                 await this.setProperties_(mediaState, scaledMediaDecodeTime,
                     lastAppendWindowStart, lastAppendWindowEnd, reference);
               }
-            });
-      }
-
-      if (shouldFixTimestampOffset) {
-        parser.parse(segment, /* partialOkay= */ false, isChunkedData);
+            })
+            .parse(segment, /* partialOkay= */ false, isChunkedData);
       }
     }