i18n [nfc]: Remove unused "Search people" 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. At that time, it was used for a button to
start a PM conversation.

In b7a2461, that single button was replaced with two buttons, one
for 1:1 PMs and one for group PMs. Those two buttons now say "New
PM" and "New group PM", and those strings are represented in
messages_en.json, as they should be.

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 people": "Search people",
   "All streams": "All streams",
   "Write a message...": "Write a message...",
   "Email": "Email",