[TASK] Simplify frontend page generation code

The additional isINTincScript check stems from the time
where this code was in a separate file, which
was included two times and page generation should be
skipped for the second inclusion. As INT scripts are only generated
during page rendering (or when fetching the page from cache)
we can safely remove this condition now.

Additionally, we can now easily get rid of the elseif
and move the page rendering preparation directly
after the same condition is checked later in the code flow.

This is fine, because this code does not need to be
executed before the lock is released. Last but not least
we clean up admin panel messages to clearly distinguish
between page generation and rendering preparation.

As this is a pure non breaking cleanup, this can
safely go into 8.7 branch as well.

Resolves: #84945
Releases: master, 8.7
Change-Id: If12b82a549d4cc47619c46f73c5423df70621b1b
Reviewed-on: https://review.typo3.org/56884


Reviewed-by: Daniel Goerz <ervaude@gmail.com>
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: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>