i18n [nfc]: Remove unused "Subscriptions" string from messages_en.json

`git grep` confirms that this string doesn't actually appear in UI
code. See the investigation that flagged this string:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/Strings.20in.20messages_en.2Ejson.20but.20not.20in.20the.20app/near/1425314

The string looks to have been present in some version of this file
since commit ab19d1b. But it stopped appearing in a UI component
in 9fd9162.

…And the latter commit even says the UI component wasn't actually
getting used at the time.

Author: chrisbobbe

static/translations/messages_en.json

@@ -108,7 +108,6 @@
   "Welcome": "Welcome",
   "Enter your Zulip server URL: <z-link>(What’s this?)</z-link>": "Enter your Zulip server URL: <z-link>(What’s this?)</z-link>",
   "Suggestion: <z-link>{suggestedServerUrl}</z-link>": "Suggestion: <z-link>{suggestedServerUrl}</z-link>",
-  "Subscriptions": "Subscriptions",
   "Search": "Search",
   "Log in": "Log in",
   "Enter": "Enter",