i18n [nfc]: Remove unused "Search messages" 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 the command

  git grep 'Search messages' ab19d1b

shows that the string didn't even appear in UI code at that
(ancient) commit.

Author: chrisbobbe

static/translations/messages_en.json

@@ -115,7 +115,6 @@
   "Log out": "Log out",
   "This will log out {email} on {realmUrl} from the mobile app on this device.": "This will log out {email} on {realmUrl} from the mobile app on this device.",
   "Add new account": "Add new account",
-  "Search messages": "Search messages",
   "Search people": "Search people",
   "All streams": "All streams",
   "Write a message...": "Write a message...",