[!!!][TASK] Remove forced STI settings from extbase configuration

The tx_extbase_type setting for fe_users and fe_groups was introduced
as a demonstration of Single Table Inheritance handled in extbase.
The forced setting brings more problems then doing good in real live
scenarios though.

So the typoscript is dropped, but TCA and sql kept for backwards
compatibility reasons.

Before 6.1 only fe_users with tx_extbase_type
Tx_Extbase_Domain_Model_FrontendUser and
fe_groups with Tx_Extbase_Domain_Model_FrontendUserGroup
are returned. Since 6.1 all fe_users and fe_groups are 
returned if you don't set the TypoScript setting on your own.

Resolves: #11021
Releases: 6.1
Change-Id: I8c1fbc48d4ee5ea1202ac6d7837170ff74eba442
Reviewed-on: https://review.typo3.org/17879
Reviewed-by: Oliver Klee
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring