What all is required to completely uninstall Webform?
Like I said you need to uninstall it from /modules UI and then delete its folder from /site/all/modules.

If you take a look at webform module source code, in webform.install under function webform_uninstall() you will find this code:

foreach (node_type_get_names() as $type =&gt; $name) {
    variable_del('webform_node_' . $type);
}


as you can see it deletes every node which has webform type. for more details of what is going to be deleted after uninstalling the module, check the  function webform_install() and function webform_uninstall() functions in the mentioned file.