How to remove custom style from source?

This issue has been around at least since 2010. Though you can override the style in other ways, the easiest way (other than avoiding use of the Recent Comments Widget) just to remove it is to add the following code to your functions.php file:

add_filter( 'show_recent_comments_widget_style', '__return_false', 99 );

see: https://core.trac.wordpress.org/ticket/11928#comment:11

Tested and works.