[!!!][TASK] Remove deprecated code from Bootstrap

Remove code that is not needed due to the static bootstrap
and PSR-15 refactoring during v9 phase.

All deprecated methods and state is removed from the Bootstrap class,
which includes the Bootstrap instance itself. Bootstrap is provided as
statically invoked utility now.
The Bootstrap instance was deprecated implictly due to the deprecation
of getInstance() (combined with the protected constructor).
We therefore remove the constructor and all (now unused) class properties
and the method-chaining (which was only useful to reduce calls to the
getInstance() method).

Releases: master
Resolves: #87202
Change-Id: Ie97e824294a03d8de88430772fdab4adaf7e3f1f
Reviewed-on: https://review.typo3.org/59196


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>