Remove prefix on comment bundles names

Before #470242: Namespacing for bundle names got fixed, we had to add a prefix to comment bundles names to differentiate between bundle 'article' on nodes and bundle 'article' on comments. This can now go away.

No UI for fields on comments yet, so this has to be tested either through #538164: Comment body as field or by running the following code in a devel PHP block.

$field = array(
  'field_name' => 'field_test_comment',
  'cardinality' => FIELD_CARDINALITY_UNLIMITED,
  'type' => 'text',
);
$instance = array(
  'field_name' => 'field_test_comment',
  'object_type' => 'comment',
  'bundle' => 'article',
  'label' => 'Multiple',
);
field_create_field($field);
field_create_instance($instance);