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/path.py

@@ -183,8 +183,7 @@ class PathFormat():
             return self.check_file()
         return False
 
-    @staticmethod
-    def check_file():
+    def check_file(self):
         return True
 
     def _enum_file(self):