Remove support for pluralisation during gettext resource translat…

…ion (#10979)

Code related to ``ngettext`` (support for n-ary / plural forms of
translated text) doesn't appear to be used, so this commit cleans up
the code by removing the ``ngettext`` reference.

Detail
------

- ``ngettext`` allows an application to provide a dynamic number
  (often a count of items, like '5' in ``the linter produced 5
  warnings``) to the translation query so that a plurality-relevant
  message can be retrieved from the catalogue.
- ``sphinx`` has previously used this within its own codebase, but no
  longer does -- ``babel`` command-line extraction [1] is used to
  retrieve translatable resources from the codebase, and extraction
  of plural-form keywords is not configured (nor easy to enable, as
  far as I could tell).
- As a result it seems like it may make sense to remove the code.

[1]: https://babel.pocoo.org/en/latest/cmdline.html#extract

sphinx/locale/__init__.py

@@ -221,16 +221,13 @@ def setup(app):
 
     .. versionadded:: 1.8
     """
-    def gettext(message: str, *args: Any) -> str:
+    def gettext(message: str) -> str:
         if not is_translator_registered(catalog, namespace):
             # not initialized yet
             return _TranslationProxy(_lazy_translate, catalog, namespace, message)  # type: ignore[return-value]  # noqa: E501
         else:
             translator = get_translator(catalog, namespace)
-            if len(args) <= 1:
-                return translator.gettext(message)
-            else:  # support pluralization
-                return translator.ngettext(message, args[0], args[1])
+            return translator.gettext(message)
 
     return gettext