Remove test_folder configuration

It's not used.

radicale/tests/custom/storage.py

@@ -29,5 +29,3 @@ class Collection(storage.Collection):
     """Collection stored in a folder."""
     def __init__(self, path, principal=False):
         super().__init__(path, principal)
-        self._filesystem_path = storage.path_to_filesystem(
-            self.configuration.get("storage", "test_folder"), self.path)

radicale/tests/test_base.py

@@ -654,7 +654,6 @@ class TestCustomStorageSystem(BaseRequests, BaseTest):
         super().setup()
         self.colpath = tempfile.mkdtemp()
         self.configuration.set("storage", "filesystem_folder", self.colpath)
-        self.configuration.set("storage", "test_folder", self.colpath)
         self.application = Application(self.configuration, self.logger)
 
     def teardown(self):