Get rid of unused var causing E_NOTICE

cacheTime is unknown in TreeRepository, and is not accepted as argument in
BufferedCache::merge.
Probably a leftover from one of the earlier patches in the cache-time commit,
where BufferedCache's methods accepted a cacheTime argument and all callers set
it to that cacheTime.

Notice: Undefined property: Flow\Repository\TreeRepository::$cacheTime in includes/Repository/TreeRepository.php on line 133

Change-Id: I5ebadc32fc75f7eb051abb1266464b9a0d2b419d
Merge "Get rid of unused var causing E_NOTICE"