Remove stale processing and reference to form_type_TYPE_value

Problem/Motivation

#2311393: Remove hook_element_info() and all references to it replaced hook_element_info with plugins, but there's a todo left in the code

// @todo Once all elements are converted to plugins in
      //   https://www.drupal.org/node/2311393, rely on
      //   $element['#value_callback'] directly.

Proposed resolution

Remove dynamic value callback of form form_type_TYPE_value

Remaining tasks

Review

User interface changes

API changes

Data model changes

Release notes snippet