[TASK] Remove unused property "cache_getRootline" in PageRepository

The protected property $cache_getRootLine is not in use anymore
and can safely be removed.

Resolves: #81101
Releases: master, 8.7
Change-Id: I4a91ed5ad9d3ae188063a060252d3da34c69abca
Reviewed-on: https://review.typo3.org/52671


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>