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/artstation.py

@@ -87,8 +87,7 @@ class ArtstationExtractor(Extractor):
 
                     yield Message.Url, url, asset
 
-    @staticmethod
-    def _image_fallback(lhs, rhs):
+    def _image_fallback(self, lhs, rhs):
         yield lhs + "/large/" + rhs
         yield lhs + "/medium/" + rhs
         yield lhs + "/small/" + rhs
@@ -172,8 +171,7 @@ class ArtstationExtractor(Extractor):
             url, method="POST", headers=headers, json={},
         ).json()["public_csrf_token"]
 
-    @staticmethod
-    def _no_cache(url):
+    def _no_cache(self, url):
         """Cause a cache miss to prevent Cloudflare 'optimizations'
 
         Cloudflare's 'Polish' optimization strips image metadata and may even
@@ -344,8 +342,7 @@ class ArtstationChallengeExtractor(ArtstationExtractor):
                     text.nameext_from_url(url, update)
                     yield Message.Url, self._no_cache(url), update
 
-    @staticmethod
-    def _id_from_url(url):
+    def _id_from_url(self, url):
         """Get an image's submission ID from its URL"""
         parts = url.split("/")
         return text.parse_int("".join(parts[7:10]))