doc: use builtin confval directive

Since version 7.4, Sphinx had a standard `confval` directive:
https://www.sphinx-doc.org/en/master/usage/domains/standard.html#directive-confval
So we can drop our custom one.

Author: bluetech

doc/en/conf.py

@@ -283,13 +283,6 @@ def setup(app: sphinx.application.Sphinx) -> None:
         indextemplate="pair: %s; fixture",
     )
 
-    app.add_object_type(
-        "confval",
-        "confval",
-        objname="configuration value",
-        indextemplate="pair: %s; configuration value",
-    )
-
     app.add_object_type(
         "globalvar",
         "globalvar",