people: Clean up recip.id code.

When we are pulling data from message.display_recipient
for private messages, the user_id field is always
called 'id', not 'user_id', so we can simplify
some defensive code.

frontend_tests/node_tests/people_errors.js

@@ -106,7 +106,7 @@ run_test('blueslip', () => {
         display_recipient: [
             {id: maria.user_id},
             {id: 42},
-            {user_id: charles.user_id},
+            {id: charles.user_id},
         ],
         sender_id: charles.user_id,
     };