[BUGFIX] Hide preview link on delete page in workspaces mode

If a page is marked to be deleted (delete placeholder) in the
workspace mode, offering a frontend preview does not make much
sense. Thus, the accordant button in the page module won't be
shown in that case.

Resolves: #65699
Releases: master, 6.2
Change-Id: Ib27721c5e4b52dc91872ddb2a2f494649e13dab5
Reviewed-on: http://review.typo3.org/37793


Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>