Widgets: Remove extra quotes from widget title in `WP_Widget_RSS`, accidentally added in [33814].

Props ShinichiN.
Fixes #34978 for trunk.
Built from https://develop.svn.wordpress.org/trunk@35978


git-svn-id: https://core.svn.wordpress.org/trunk@35943 1a063a9b-81f0-0310-95a4-ce76da25c4cd