DEV: remove targetUsernames deprecation

Stable already has this rename, no need to carry the deprecation anymore.

app/assets/javascripts/discourse/models/composer.js

@@ -77,8 +77,6 @@ const CLOSED = "closed",
     composerTime: "composerTime",
     typingTime: "typingTime",
     postId: "post.id",
-    // TODO remove together with 'targetUsername' deprecations
-    usernames: "targetUsernames",
     recipients: "targetRecipients"
   },
   _add_draft_fields = {},
@@ -342,14 +340,6 @@ const Composer = RestModel.extend({
     return options;
   },
 
-  @discourseComputed("targetRecipients")
-  targetUsernames(targetRecipients) {
-    deprecated(
-      "`targetUsernames` is deprecated, use `targetRecipients` instead."
-    );
-    return targetRecipients;
-  },
-
   @discourseComputed("targetRecipients")
   targetRecipientsArray(targetRecipients) {
     const recipients = targetRecipients ? targetRecipients.split(",") : [];