[TASK] Remove deprecated classes from t3lib

Some classes in t3lib were not moved to namespaces and
deprecated with 6.0. Those are now removed:

* t3lib_ajax is scheduled for removal and unused
* t3lib_install is only referenced in dbal from an unused hook,
  cleanup for dbal will follow
* t3lib_readmail is scheduled for removal and unused
* t3lib_syntaxhl is scheduled for removal and unused
* t3lib_userauthgroup is scheduled for removal and unused
* t3lib_xml is scheduled for removal and unused
* t3lib_http_request is scheduled for removal and unused, the
  additional require_once is included in the new class location
* t3lib_mail_mailer is scheduled for removal and unused
* t3lib_mail_message is scheduled for removal and unused, the
  additional require_once is included in the new class location

Change-Id: Ic7688d89e62c659000f81a53b47beaf73e455ada
Resolves: #47761
Releases: 6.2
Reviewed-on: https://review.typo3.org/20364
Reviewed-by: Thomas Maroschik
Tested-by: Thomas Maroschik
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn