[BUGFIX] Disallow deleting extensions in Composer mode

Deleting an extension in Extension Manager doesn't make much sense in a
Composer-based installation. For this reason, the removal of extensions
is prohibited now.

Resolves: #91456
Releases: master, 9.5
Change-Id: Ia96cf2741fd749d9f50540366351c8b576cac96b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64568


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>