Remove unused deleteCookieBannerTranslation mutation

The mutation was wired up in the GraphQL schema and resolver
but never called from the console frontend. This removes the
service method, request type, resolver, schema entries, and
two unused error sentinels (ErrTranslationNotFound,
ErrTranslationAlreadyExists).

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/cookiebanner/errors.go

@@ -33,6 +33,4 @@ var (
 	ErrCookieNameAlreadyExists    = errors.New("a cookie with this name already exists in this banner")
 	ErrCategoriesBannerMismatch   = errors.New("source and target categories belong to different banners")
 	ErrSameCategoryMove           = errors.New("source and target cookie categories must be different")
-	ErrTranslationNotFound        = errors.New("cookie banner translation not found")
-	ErrTranslationAlreadyExists   = errors.New("translation for this language already exists")
 )