Functions theme_get_setting(), file_create_url() and file_url_transform_relative() are deprecated

Problem/Motivation

Functions file_create_url() and file_url_transform_relative() are deprecated in Drupal 9 version (https://www.drupal.org/node/2940031)
and function theme_get_setting() is deprecated since Drupal 8.7 version (https://www.drupal.org/node/3035289)

Proposed resolution

Remove these deprecated functions.