How to remove comment link title attribute?

The title attribute is hard-coded in comments_popup_link() unfortunately:

echo ' title="' . esc_attr( sprintf( __('Comment on %s'), $title ) ) . '">';

What you can do is catching the generated HTML in a variable and replacing the attribute with an empty string:

ob_start();
comments_popup_link();
print preg_replace( '~ title="[^"]+"~', '', ob_get_clean() );