Widget Customizer: Remove `UPDATE_WIDGET_AJAX_ACTION` and `UPDATE_WIDGET_NONCE_POST_KEY` constants.

see #27534.
Built from https://develop.svn.wordpress.org/trunk@27818


git-svn-id: http://core.svn.wordpress.org/trunk@27652 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Widget Customizer: Simplify nonces.

see #27534.
Built from https://develop.svn.wordpress.org/trunk@27819


git-svn-id: http://core.svn.wordpress.org/trunk@27653 1a063a9b-81f0-0310-95a4-ce76da25c4cd