[TASK] Remove unused TYPO3 configuration option

The t3InstId option was introduced in version 3.x and never used
by the old EM, and is not even used by the current EM, and also
not with the t3o. As long as there is no functionality
that deals with that option, it should be removed completely again.

Resolves: #46993
Releases: 6.1
Change-Id: Ic506c89375ce012c6ea2bb73649aaa50a2155104
Reviewed-on: https://review.typo3.org/19689
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Wouter Wolters
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring