Removed unused doesTranslationKeyExist fn from i18n

- With the recent removal of i18n from error handling, we've removed the usecase for this function
- Cleaning up as we slowly get rid of i18n :)

core/shared/i18n/i18n.js

@@ -128,15 +128,6 @@ class I18n {
         }
     }
 
-    /**
-     * Check if a key exists in the loaded strings
-     * @param {String} msgPath
-     */
-    doesTranslationKeyExist(msgPath) {
-        const translation = this._findString(msgPath, {log: false});
-        return translation !== this._fallbackError();
-    }
-
     /**
      * Do the lookup within the JSON file using jsonpath
      *