[TASK] Deprecate Extbase model FileMount

The Extbase model `FileMount` has never been used and there is also no
need to have a model for every table. To streamline the codebase, the
class has been marked as deprecated.

Resolves: #90686
Releases: master
Change-Id: I141a3171dd9f8c2b7d961541753653886f787d41
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63639


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christian Eßl <indy.essl@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>