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/unsplash.py

@@ -48,8 +48,7 @@ class UnsplashExtractor(Extractor):
             yield Message.Directory, photo
             yield Message.Url, url, photo
 
-    @staticmethod
-    def metadata():
+    def metadata(self):
         return None
 
     def skip(self, num):