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/foolslide.py

@@ -24,8 +24,7 @@ class FoolslideExtractor(BaseExtractor):
         return BaseExtractor.request(
             self, url, encoding="utf-8", method="POST", data={"adult": "true"})
 
-    @staticmethod
-    def parse_chapter_url(url, data):
+    def parse_chapter_url(self, url, data):
         info = url.partition("/read/")[2].rstrip("/").split("/")
         lang = info[1].partition("-")[0]
         data["lang"] = lang