I don't know of a way to do this without hacking the function (a tiny bit). If you're up to it:
list_cats() is in template-functions-category.php (wp-includes/ directory). The function starts around line 248; the line we're looking for is at about line 315:
$link .= 'title="'. sprintf(__("View all posts filed under %s"), wp_specialchars($category->cat_name)) . '"';
Just comment it out:
// $link .= 'title="'. sprintf(__("View all posts filed under %s"), wp_specialchars($category->cat_name)) . '"';
Hacking notes: Back up source files before editing them, and comment your changes for future reference.