emoji: Remove `display_url` from the context of `admin_emoji_list`.

This attribute seem to be artifact of some refactoring that we did
but forgot to remove it. Just a minor cleanup.

frontend_tests/node_tests/dispatch.js

@@ -272,7 +272,7 @@ var event_fixtures = {
         type: 'realm_emoji',
         realm_emoji: {
             airplane: {
-                display_url: 'some_url',
+                source_url: 'some_url',
             },
         },
     },

frontend_tests/node_tests/templates.js

@@ -172,7 +172,6 @@ run_test('admin_emoji_list', () => {
         emoji: {
             name: "MouseFace",
             display_name: "MouseFace",
-            display_url: "http://emojipedia-us.s3.amazonaws.com/cache/46/7f/467fe69069c408e07517621f263ea9b5.png",
             source_url: "http://emojipedia-us.s3.amazonaws.com/cache/46/7f/467fe69069c408e07517621f263ea9b5.png",
         },
     };

static/js/settings_emoji.js

@@ -67,7 +67,6 @@ exports.populate_emoji = function (emoji_data) {
                     name: data.name,
                     display_name: data.name.replace(/_/g, ' '),
                     source_url: data.source_url,
-                    display_url: data.source_url,
                     author: data.author || '',
                     can_admin_emoji: can_admin_emoji(data),
                 },