[TASK] Remove extended fluid core class CmsVariableProvider

The CmsVariableProvider within EXT:fluid extends a Fluid core class with
the same functionality and can be removed. A class alias ensures that the
PHP class is still available.

Resolves: #82229
Releases: master
Change-Id: I71eb7beaf0684631d9c7e857bbed91f08b75d47c
Reviewed-on: https://review.typo3.org/53227


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Claus Due <claus@phpmind.net>
Tested-by: Claus Due <claus@phpmind.net>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>