[TASK] Remove t3lib_superadmin

The class is obsolete since 4.5 and can be removed now.

Change-Id: I914e3ef199c2215ca25ffa27631ac28ecf05ac50
Resolves: #31299
Releases: 4.7
Reviewed-on: http://review.typo3.org/6340
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Oliver Klee
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn