use built-in clear-block CSS class for clearing in user profile lists [#143200]

The function theme_user_profile creates DIVs with class 'profile', and then uses a clear:both; rule in modules/user/user.css to clear any floats (eg profile pictures).
A better approach is to add the CSS clear-block class which is defined in Drupal core. In other words, change:
$output = "

\n";
to
$output = "

\n";

This has the advantage of extending the DIV's bounding box to include all floats. In other words, the border can be styled. Currently, it will cut through floated profile images.
The clear: both; rule for .profile can also be dropped from modules/user/user.css.