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/furaffinity.py

@@ -152,8 +152,7 @@ class FuraffinityExtractor(Extractor):
 
         return data
 
-    @staticmethod
-    def _process_description(description):
+    def _process_description(self, description):
         return text.unescape(text.remove_html(description, "", ""))
 
     def _pagination(self, path, folder=None):