Remove deprecated AllowedTagsXssTrait from ListItemBase

The AllowedTagsXssTrait is deprecated in favor of FieldFilteredMarkup and will be removed in Drupal 9.

We need to:

1. Remove the Use statements for the trait from core code. (see #6 and #13)
2. Replace calls to AllowedTagsXssTrait::fieldFilterXss() with FieldFilteredMarkup::create()
3. Replace calls to AllowedTagsXssTrait::allowedTags() with FieldFilteredMarkup::allowedTags()
4. Replace calls to AllowedTagsXssTrait::displayAllowedTags() with FieldFilteredMarkup::displayAllowedTags()

#17 takes care of the above and solves the test failures. dmitryl is re-rolling a patch that removes the trait's php file.