i18n [nfc]: Remove unused "Write a message..." 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 this file since commit
ab19d1b.

…but, huh, seems like it was never actually used in the UI. The
following command comes up empty:

  git log -S 'Write a message...' -- . ':!static/translations' ':!src/i18n/translations'

i.e., show commits that added or removed occurrences of the string,
ignoring changes in `static/translations` and
`src/i18n/translations` (the latter being the old path before it was
changed to the former).

Author: chrisbobbe

static/translations/messages_en.json

@@ -116,7 +116,6 @@
   "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",
   "All streams": "All streams",
-  "Write a message...": "Write a message...",
   "Email": "Email",
   "Username": "Username",
   "Password": "Password",