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/wallhaven.py

@@ -39,8 +39,7 @@ class WallhavenExtractor(Extractor):
         """Return general metadata"""
         return ()
 
-    @staticmethod
-    def _transform(wp):
+    def _transform(self, wp):
         wp["url"] = wp.pop("path")
         if "tags" in wp:
             wp["tags"] = [t["name"] for t in wp["tags"]]