Disable translation_progress_classes

Sphinx does not properly convert command-line overrides
to Boolean values, so ``-D translation_progress_classes=1``
fails with:

    Sphinx parallel build error:
    sphinx.errors.ConfigError: translation_progress_classes must be True, False, "translated" or "untranslated"

build_docs.py

@@ -710,7 +710,7 @@ def build(self):
                     f"-D locale_dirs={locale_dirs}",
                     f"-D language={self.language.iso639_tag}",
                     "-D gettext_compact=0",
-                    "-D translation_progress_classes=1",
+                    # "-D translation_progress_classes=1",
                 )
             )
         if self.language.tag == "ja":