Themes: Remove debug call from `wp-admin/theme-install.php`.

Follow-up to [47816].

See #48491.
Built from https://develop.svn.wordpress.org/trunk@47817


git-svn-id: https://core.svn.wordpress.org/trunk@47593 1a063a9b-81f0-0310-95a4-ce76da25c4cd