yarn: Remove i18n packages that are no longer used.

We removed these when we converted the app's frontend translations
mechanism to instead send the translation data in `page_params`.

package.json

@@ -23,10 +23,6 @@
     "fuzzysearch": "1.0.3",
     "handlebars": "4.0.11",
     "i18next": "3.4.4",
-    "i18next-browser-languagedetector": "0.3.0",
-    "i18next-localstorage-cache": "0.3.0",
-    "i18next-parser": "0.11.1",
-    "i18next-xhr-backend": "1.5.1",
     "jquery": "3.3.1",
     "jquery-validation": "1.17.0",
     "katex": "0.8.3",