Docs: Remove unused `###USERNAME###` placeholder reference from `user_request_action_email_content` filter documentation.

Props desrosj.
Merges [43189] to the 4.9 branch.
Fixes #44016.
Built from https://develop.svn.wordpress.org/branches/4.9@43190


git-svn-id: http://core.svn.wordpress.org/branches/4.9@43019 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -3034,7 +3034,7 @@ All at ###SITENAME###
 	 * Filters the text of the email sent when an account action is attempted.
 	 *
 	 * The following strings have a special meaning and will get replaced dynamically:
-	 * ###USERNAME###           The user's username, if the user has an account. Prefixed with single space. Otherwise left blank.
+	 *
 	 * ###DESCRIPTION### Description of the action being performed so the user knows what the email is for.
 	 * ###CONFIRM_URL### The link to click on to confirm the account action.
 	 * ###EMAIL###       The email we are sending to.

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9.6-beta1-43188';
+$wp_version = '4.9.6-beta1-43190';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.