[!!!][BUGFIX] Avoid unnecessary de-/serialization for page cache

When storing a cache in page the array variable "cache_data" gets
serialized and when being retrieved from cache unserialized.

This is not necessary as de-/serialization is already performed by
the variable caching backend.

Change-Id: I9cf817123947af3f4076dabdce3c569bf41b477c
Resolves: #55493
Releases: 6.2
Reviewed-on: https://review.typo3.org/27179
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf