It looks like I have solved my own problem. I’m guessing that a function like that creates a permanent change, regardless of whether the function remains in the functions.php file. So, to fix this, I just added a new function reversing this adding. Here is that function:
function remove_theme_caps() {
$role = get_role( 'author' );
$role->remove_cap( 'edit_others_posts' );
}
add_action( 'admin_init', 'remove_theme_caps');
This has restored the default setting of not allowing authors to edit other users’ posts. And after uploading my functions file with this code, I have since deleted this section of code and re-uploaded my functions file, and the change (back to the default setting) has remained. So, problem solved!