Cleanup `wp_widget_rss_form()` after [28734]. "$$input used sanitized variables which contained actual values, unlike $inputs[$input] which in that context contains data about which input fields are hidden."

Props kovshenin.
Fixes #27881.

Built from https://develop.svn.wordpress.org/trunk@28787


git-svn-id: http://core.svn.wordpress.org/trunk@28600 1a063a9b-81f0-0310-95a4-ce76da25c4cd