[!!!][TASK] Drop xcache cache backend

Xcache does not support PHP 7 and there seems to be little
interest in making it compatible. For a server local memory
cache backend apcu can be used as alternative.
The patch drops the xcache cache backend in master since
it doesn't work anyway.

Change-Id: I850e58889520aa394b427fb9c52efdbed72758c4
Resolves: #76802
Releases: master
Reviewed-on: https://review.typo3.org/48694


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>