chore(i18n): drop 4 orphan keys from the mn locale

mn/common.json carried 4 keys absent from the en reference
(login.setup_guide, language.mn, language.mongolian,
language.switch_to_mongolian), failing the key-parity test. They are unused in
source and not present in any other locale (Mongolian is not wired into the
language switcher in en), so remove them to restore parity.

translations.test green (50).

locales/mn/common.json

@@ -46,7 +46,6 @@
     "cancel": "Болих",
     "website": "Вэбсайт",
     "imprint": "Хуулийн мэдээлэл",
-    "setup_guide": "Мэйл тохируулах",
     "privacy_policy": "Нууцлалын бодлого",
     "try_demo": "Демо үзэх",
     "demo_description": "Жишээ өгөгдлөөр туршиж үзээрэй - бүртгэл шаардлагагүй",
@@ -846,7 +845,6 @@
     "ja": "日本語",
     "ko": "한국어",
     "lv": "Латвиешу",
-    "mn": "Монгол",
     "nl": "Нидерланд",
     "pl": "Полски",
     "pt": "Португали",
@@ -857,8 +855,6 @@
     "zh": "简体中文",
     "fa": "فارسی",
     "farsi": "فارسی",
-    "mongolian": "Монгол",
-    "switch_to_mongolian": "Монгол хэл рүү шилжих",
     "switch_to_farsi": "Фарси хэл рүү шилжих"
   },
   "settings": {