[TASK] Remove deprecated getCount in FolderTreeView

Resolves: #56840
Releases: 6.2
Change-Id: I7a09565946663fe8ca1e3d1c88c74f5c4ac1975e
Reviewed-on: https://review.typo3.org/28318
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer