comments_template(): Use TwentyTen comments.php file instead of Default

[zeo]

If the current activated theme's comments.php file is not found, use twentyten's comments.php. Now that the Default theme have been completely wipe out.

use TwentyTen comments.php in comments_template()

If the Default (Kubrick) theme is present, its comments.php should continue to be used on existing installs. That is, don't change things on existing installs.

See ticket:12846