messageList: Remove secondary actor API from Message

The secondary actor isn't used for anything anymore.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3103>

data/theme/gnome-shell-sass/widgets/_message-list.scss

@@ -99,18 +99,14 @@
       font-weight: bold;
     }
 
-    // secondary container in title box
-    .message-secondary-bin {
+    // notification time stamp
+    > .event-time {
+      @extend %caption;
+      color: $insensitive_fg_color;
+      padding-bottom: to_em(1px);
 
-      // notification time stamp
-      > .event-time {
-        @extend %caption;
-        color: $insensitive_fg_color;
-        padding-bottom: to_em(1px);
-
-        &:ltr { text-align: right };
-        &:rtl { text-align: left };
-      }
+      &:ltr { text-align: right };
+      &:rtl { text-align: left };
     }
   }
 

js/ui/messageList.js

@@ -409,12 +409,8 @@ export const Message = GObject.registerClass({
         this.setTitle(title);
         titleBox.add_child(this.titleLabel);
 
-        this._secondaryBin = new St.Bin({
-            style_class: 'message-secondary-bin',
-            x_expand: true, y_expand: true,
-            y_align: Clutter.ActorAlign.END,
-        });
-        titleBox.add_child(this._secondaryBin);
+        this._timeLabel = new TimeLabel();
+        titleBox.add_child(this._timeLabel);
 
         this._closeButton = new St.Button({
             style_class: 'message-close-button',
@@ -450,21 +446,11 @@ export const Message = GObject.registerClass({
     }
 
     get datetime() {
-        if (this._secondaryBin.child instanceof TimeLabel)
-            return this._secondaryBin.child.datetime;
-        else
-            return null;
+        return this._timeLabel.datetime;
     }
 
     set datetime(datetime) {
-        if (!(this._secondaryBin.child instanceof TimeLabel))
-            this._secondaryBin.child = new TimeLabel();
-
-        this._secondaryBin.child.set({datetime});
-    }
-
-    setSecondaryActor(actor) {
-        this._secondaryBin.child = actor;
+        this._timeLabel.datetime = datetime;
     }
 
     setTitle(text) {

js/ui/mpris.js

@@ -31,7 +31,7 @@ class MediaMessage extends MessageList.Message {
         this.setIcon(this._icon);
 
         // reclaim space used by unused elements
-        this._secondaryBin.hide();
+        this._timeLabel.hide();
         this._closeButton.hide();
 
         this._prevButton = this.addMediaControl('media-skip-backward-symbolic',