[TASK] Cleanup ext_localconf.php of sysexts

As TYPO3 is loading all ext_localconf.php on each call,
the existing code can be cleaned up by removing
old information, putting BE-only functionality in
TYPO3_MODE == 'BE' and install-tool only
in Install only mode.

Also, some hardcoded require_once statements are
removed.

Benefits are that FE does not need to load everything
all the time.

I haven't noticed any side-effects so far, but please
test extensively.

Resolves: #55593
Releases: 6.2
Change-Id: Ibc37e84cb2af05ddb6b227290330aad31037a018
Reviewed-on: https://review.typo3.org/27286
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters