[BUGFIX] Remove unnecessary and failing code

While fixing #57199 a slot was introduced to write the class cache
after each extension installation. This not only adds unnecessary overhead
as the class loader already holds the class loading information of activated
packages in runtime cache, it currently causes fatal errors in certain
situations, as the class cache is also removed after each
extension installation *and* runtime information is flushed in the slot.

Removing the slot increases stability and performance
for extension installation process.

Resolves: #66487
Releases: 6.2, master
Change-Id: Ie33a73af7abb8aa50cd30cbfa30bfb43031f0cea
Reviewed-on: http://review.typo3.org/38821


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>