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/reddit.py

@@ -573,8 +573,7 @@ class RedditAPI():
         sid = self.extractor.config(key)
         return self._decode(sid.rpartition("_")[2].lower()) if sid else default
 
-    @staticmethod
-    def _decode(sid):
+    def _decode(self, sid):
         return util.bdecode(sid, "0123456789abcdefghijklmnopqrstuvwxyz")