Fix path resolution during 'sphinx.cmd.make_mode' cleanup (#13158)

Author: jayaddison

sphinx/cmd/make_mode.py

@@ -87,7 +87,7 @@ def build_clean(self) -> int:
             return 1
         print("Removing everything under '%s'..." % self.build_dir)
         for item in self.build_dir.iterdir():
-            rmtree(self.build_dir_join(item))
+            rmtree(item)
         return 0
 
     def build_help(self) -> None: