remove @staticmethod decorators

There might have been a time when calling a static method was faster
than a regular method, but that is no longer the case. According to
micro-benchmarks, it is 70% slower in CPython 3.13 and it also makes
executing the code of a class definition slower.

gallery_dl/extractor/vichan.py

@@ -73,8 +73,7 @@ class VichanThreadExtractor(VichanExtractor):
             self.root, post["board"], post["tim"], post["ext"])
         return Message.Url, post["url"], post
 
-    @staticmethod
-    def _process_8kun(post, data):
+    def _process_8kun(self, post, data):
         post.update(data)
         post["extension"] = post["ext"][1:]