Remove @deprecated _drupal_add_http_header() and friends.

Problem/Motivation

@deprecated things are deprecated, including _drupal_add_http_header() and other header-related functions from bootstrap.inc.

HTTP headers are now handed by the Symfony Request/Response objects. Let's finish the job of encapsulating responses by removing the global header-related functions.

The affected functions include:

• drupal_set_preferred_header_name()
• drupal_add_http_header() - Depends on the status of drupal_process_attached() #2382545: Cleanup drupal_process_attached and _drupal_add_* documentation
• drupal_get_http_header()
• drupal_page_header()
• drupal_send_headers()

Note that FinishResponseSubscriber calls drupal_get_http_header(), in order to retain backward compatibility, which will be obsoleted by this patch.

Proposed resolution

Remove these functions from Drupal core.

Remaining tasks

User interface changes

API changes

Beta phase evaluation