[!!!][TASK] Remove request type specific code in Bootstrap

As all request type specific initialization has been
refactored to the Application classes, request type
specific code in Bootstrap is unused [1] and can
be removed.

[1] redirectToInstallTool() is actually still in use, but moved
into the frontend and backend Application classes with this change.
(Also make their parent class abstract as drive-by change.)

Change-Id: Ieded3603c2936d83d3d4701abef6c3884b558e31
Releases: master
Resolves: #83869
Reviewed-on: https://review.typo3.org/55692


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>