[TASK] Remove unused ThumbnailController

The `@internal` `ThumbnailController` is removed,
because it has been unused since the introduction
of the deferred BE image processing in #92188.

Resolves: #101992
Related: #92188
Releases: main
Change-Id: I670acb83fcc7be9c8867658467b4e1ca87bcb954
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81152


Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>