[BUGFIX] Hide move placeholder in WS preview

Move placeholder are visible in the navigation
during WS preview if they are moved for example
from the first to the second level.

Change-Id: I7e3339488d9d64cdf04154fb0334880f76576f8d
Fixes: #38705
Releases: 6.0,4.7,4.6,4.5
Reviewed-on: https://review.typo3.org/12666
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack