[TASK] Remove deprecated code for 6.0

Removes all deprecated methods and files that are unused in core and scheduled
for deletion in 4.8 / 6.0. Some more complicated parts are left out for now.

Change-Id: I03e236cedfbfa666d9b6f83848b11bb0977cdff0
Resolves: #35923
Releases: 6.0
Reviewed-on: http://review.typo3.org/10361
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn