remove unused code

Author: wenshao

core/src/main/java/com/alibaba/fastjson2/JSONReaderUTF8.java

@@ -4733,7 +4733,6 @@ public final void skipComment() {
 
         final byte[] bytes = this.bytes;
         byte ch = bytes[offset++];
-        byte start = ch;
 
         boolean multi;
         if (ch == '*') {