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
James Addison